
Qass QB4,4i 
Book-^.„D7 



ASTRONOMY OF TO-DAY 



ASTRONOMY OF 
TO-DAY 

A T OVULAR INTRODUCTION IN 
NON-TECHNICAL LANGUAGE 



By 
CECIL G. DOLMAGE, M.A., LL.D., D.C.L. 

Fellow of the Royal Astronomical Society ; Member of 

the British Astronomical Association ; Member of 

the Astronomical Society of the Pacific ; Membre 

de la Societe Astronomique de France; 

Membre de la Societe Beige 

d'Astronomie 



With a Frontispiece in Colour 
and 45 Illustrations ^ Diagrams 



PHILADELPHIA 
B. LIPPINCOTT COMPANY 

LONDON : SEELEY & CO. Limited 
1909 



»>^ 

<^.^^ 



/f 3/3^1 



7 



PREFACE 



The object of this book is to give an account of the 
science of Astronomy, as it is known at the present 
day, in a manner acceptable to \hQ general reader. 

It is too often supposed that it is impossible to 
acquire any useful knowledge of Astronomy without 
much laborious study, and without adventuring into 
quite a new world of thought. The reasoning apphed 
to the study of the celestial orbs is, however, of no 
different order from that which is employed in the 
affairs of everyday life. The science of mathematics 
is perhaps responsible for the idea that some kind of 
difference does exist ; but mathematical processes 
are, in effect, no more than ordinary logic in con- 
centrated form, the shorthand of reasoning ^ so to speak. 
I have attempted in the following pages to take the 
main facts and theories of Astronomy out of those 
mathematical forms which repel the general reader, 
and to present them in the ordinary language of our 
workaday world. 

The few diagrams introduced are altogether supple- 
mentary, and are not connected with the text by any 
wearying cross-references. Each diagram is com- 
plete in itself, being intended to serve as a pictorial 
aid, in case the wording of the text should not have 
perfectly conveyed the desired meaning. The full 

V 



Preface 

page illustrations are also described as adequately 
as possible at the foot of each. 

As to the coloured frontispiece, this must be placed 
in a category by itself. It is the work of the artist 
as distinct from the scientist. 

The book itself contains incidentally a good deal 
of matter concerned with the Astronomy of the past, 
the introduction of which has been found necessary 
in order to make clearer the Astronomy of our time. 

It would be quite impossible for me to enumerate 
here the many sources from which information has 
been drawn. But I acknowledge my especial in- 
debtedness to Professor F, R. Moulton's Introduction 
to Astronomy (Macmillan, 1906), to the works on 
EcHpses of the late Rev. S. ]. Johnson and of Mr. 
W. T. Lynn, and to the excellent Journals of the 
British Astronomical Association, Further, for those 
grand questions concerned with the Stellar Universe 
at large, I owe a very deep debt to the writings of 
the famous American astronomer, Professor Simon 
Newcomb, and of our own countryman, Mr. John 
Ellard Gore ; to the latter of whom I am under an 
additional obligation for much valuable information 
privately rendered. 

In my search for suitable illustrations, I have 
been greatly aided by the kindly advice of Mr. W. 
H. Wesley, the Assistant Secretary of the Royal 
Astronomical Society. To those who have been so 
good as to permit me to reproduce pictures and 
photographs, I desire to record my best thanks as 
follows : — To the French Artist, Mdlle. Andree Moch ; 
to the Astronomer Royal ; to Sir David Gill, K.GB., 
LL.D., F.R.S. ; to the Council of the Royal Astrono- 

vi 



Preface 

mical Society ; to Professor E. B. Frost, Director of 
the Yerkes Observatory ; to M. P. Puiseux, of the 
Paris Observatory ; to Dr. Max Wolf, of Heidelberg ; 
to Professor Percival Lowell ; to the Rev. Theodore 
E. R. PhilUps, M.A., F.R.A.S. ; to Mr. W. H. Wesley ; 
to the Warner and Swasey Co., of Cleveland, Ohio, 
U.S.A. ; to the publishers of Knowledge, and to Messrs. 
Sampson, Low & Co. For permission to reproduce 
the beautiful photograph of the Spiral Nebula in 
Canes Venatici (Plate XXIL), I am indebted to the 
distinguished astronomer, the late Dr. W. E. Wilson, 
D.Sc, F.R.S., whose untimely death, I regret to state, 
occurred in the early part of this year. 

Finally, my best thanks are due to Mr. John Ellard 
Gore, F.R.A.S., M.R.LA., to Mr. W. H. Wesley, and 
to Mr. John Butler Burke, M.A., of Cambridge, for 
their kindness in reading the proof-sheets. 

CECIL G. DOLMAGE. 

London, S.W., 

August 4, 1908. 



vu 



CONTENTS 



CHAPTER I 

PAGE 

The Ancient View 17 



CHAPTER II 
The Modern View 20 

CHAPTER III 
The Solar System 29 

CHAPTER IV 
Celestial Mechanism 38 

CHAPTER V 
Celestial Distances . . . . . 46 

CHAPTER VI 

Celestial Measurement 55 

ix 



Contents 

CHAPTER VII 



PAGE 

Eclipses and Kindred Phenomena . . 6i 



CHAPTER VIII 
Famous Eclipses of the Sun ... 83 

CHAPTER IX 
Famous Eclipses of the Moon . . .101 

CHAPTER X 
The Growth of Observation . . . 105 

CHAPTER XI 
Spectrum Analysis 121 

CHAPTER XII 

The Sun . . . . . . . .127 

CHAPTER XIII 
The Sun — continued 134 

CHAPTER XIV 
The Inferior Planets 146 

X 



Contents 

CHAPTER XV 

PAGE 

The Earth 158 

CHAPTER XVI 
The Moon 183 

CHAPTER XVII 
The Superior Planets . . . . . 209 

CHAPTER XVIII 
The Superior Planets — continued . . . 229 

CHAPTER XIX 
Comets . . . . . . . . 247 

CHAPTER XX 
Remarkable Comets 259 

CHAPTER XXI 
Meteors or Shooting Stars .... 266 

CHAPTER XXII 

The Stars . 278 

xi 



Contents 

CHAPTER XXIII 



PAGE 



The Stars — continued ..... 287 

CHAPTER XXIV 
Systems of Stars ...... 300 

CHAPTER XXV 
The Stellar Universe 319 

CHAPTER XXVI 
The Stellar Universe — continued . . 329 

CHAPTER XXVII 
The Beginning of Things .... 333 

CHAPTER XXVIII 
The End of Things 342 

Index 351 



Xll 



LIST OF ILLUSTRATIONS 



LIST OF PLATES 



The Total Eclipse of the Sun 
OF August 30, 1905 . 

I. The Total Eclipse of the Sun 
OF May 17, 1882 

II. Great Telescope of Hevelius 

III. A TUBELESS, OR " AeRIAL" TELE- 

SCOPE .... 

IV. The Great Yerkes Telescope 

V. The Sun, showing several 
GROUPS OF Spots 

VI. Photograph of a Sunspot 

VII. Forms of the Solar Corona 

AT THE epochs OF SUNSPOT 

Maximum and Sunspot 
Minimum respectively. 

(A) The Total Eclipse of 

THE Sun of December 
22, 1870. 

(B) The Total Eclipse of 

THE Sun of May 28, 



• 


Frontispiece 


NT 

. To face page 


96 


J3 


11 


IIO 


J? 


3) 


112 


5) 


J3 


118 


L 

• 55 


5) 


134 


)) 


)J 


136 



1900 



142 



Xlll 



List of Illustrations 

VIII. The Moon .... To face page 196 



PLATE 



IX. Map of the Moon, showing 

THE PRINCIPAL " CRATERS," 

Mountain Ranges and 

"Seas" ....„„ 198 

X. One of the most interesting 

Regions on the Moon . „ „ 200 

XI. The Moon (showing Systems 

OF " Rays ") . . . „ „ 204 

XII. A Map of the Planet Mars „ „ 216 

XIII. Minor Planet Trails . . „ „ 226 

XIV. The Planet Jupiter . . „ „ 230 

XV. The Planet Saturn . . „ „ 236 

XVI. Early Representations of 

Saturn . . . • » » 242 

XVII. DoNATi's Comet . . . „ „ 256 

XVIII. Daniel's Comet of 1907 . „ „ 258 

XIX. The Sky around the North 

Pole . . . • jj » 292 

XX. Orion and his Neighbours . ,, „ 296 

XXI. The Great Globular Cluster 
in the Southern Constel- 
lation OF Centaurus . „ „ 306 

XXII. Spiral Nebula in the Con- 
stellation OF Canes Vena- 



tici 

xiv 



314 



List of Illustrations 

PLATE 

XXIII. The Great Nebula in the 
Constellation of Andro- 
meda . . . . .To face page 316 

XXIV. The Great Nebula in the 

Constellation of Orion . ,, „ 318 



LIST OF DIAGRAMS 

FIG. PAGE 

1. The Ptolemaic Idea of the Universe . 19 

2. The Copernican Theory of the Solar 

System .21 

3. Total and Partial Eclipses of the Moon . 64 

4. Total and Partial Eclipses of the Sun . 67 

5. " Baily's Beads" 70 

6. Map of the World on Mercator's Projec- 

tion, SHOWING A portion OF THE PRO- 
GRESS OF THE Total Solar Eclipse of 
August 30, 1905, across the surface 
of the Earth . . . . .81 

7. The " Ring with Wings " . . . .87 

8. The Various Types of Telescope . . 113 

9. The Solar. Spectrum 123 

10. A Section through the Sun, showing how 

the Prominences rise from the Chro- 
mosphere . . . . . • 131 

11. Orbit and Phases of an Inferior Planet . 148 

XV 



List of Illustrations 

FIG. PAGE 

12. The "Black Drop" 153 

13. Summer and Winter .... 

14. Orbit and Phases of the Moon . 

15. The Rotation of the Moon on her Axis 

16. Laplace's "Perennial Full Moon" 



176 

184 
187 
191 



17. Illustrating the Author's explanation of 

the apparent enlargement of celestial 
Objects ....... 195 

18. Showing how the Tail of a Comet is 

directed away FROM THE SUN . . 248 

19. The Comet of 1066, as represented in the 

Bayeux Tapestry , . . . .263 

20. Passage of the Earth through the thickest 

portion of a Meteor Swarm . .269 



XVI 



ASTRONOMY OF TO-DAY 



CHAPTER I 

THE ANCIENT VIEW 

It is never safe, as we know, to judge by appearances, 
and this is perhaps more true of astronomy than of 
anything else. 

For instance, the idea which one would most 
naturally form of the earth and heaven is that the 
solid earth on which we live and move extends to a 
great distance in every direction, and that the heaven 
is an immense dome upon the inner surface of which 
the stars are fixed. Such must needs have been the 
idea of the universe held by men in the earliest times. 
In their view the earth was of paramount importance. 
The sun and moon were mere lamps for the day 
and for the night ; and these, if not gods themselves, 
were at any rate under the charge of special deities, 
whose task it was to guide their motions across the 
vaulted sky. 

Little by little, however, this simple estimate of nature 
began to be overturned. Difficult problems agitated 
the human mind. On what, for instance, did the solid 
earth rest, and what prevented the vaulted heaven from 
falling in upon men and crushing them out of exist- 

17 B 



Astronomy of To-day 

ence ? Fantastic myths sprang from the vain attempts 
to solve these riddles. The Hindoos, for example, ima- 
gined the earth as supported by four elephants which 
stood upon the back of a gigantic tortoise, which, in 
its turn, floated on the surface of an elemental ocean. 
The early Western civilisations conceived the fable 
of the Titan Atlas, who, as a punishment for revolt 
against the Olympian gods, was condemned to hold 
up the expanse of sky for ever and ever. 

Later on glimmerings of the true light began to 
break in upon men. The Greek philosophers, who 
busied themselves much with such matters, gradually 
became convinced that the earth was spherical in shape, 
that is to say, round like a ball. In this opinion we now 
know that they were right ; but in their other impor- 
tant belief, viz. that the earth was placed at the centre 
of all things, they were indeed very far from the truth. 

By the second century of the Christian era, the 
ideas of the early philosophers had become hardened 
into a definite theory, which, though it appears very 
incorrect to us to-day, nevertheless demands excep- 
tional notice from the fact that it was everywhere 
accepted as the true explanation until so late as some 
four centuries ago. This theory of the universe is 
known by the name of the Ptolemaic System, because 
it was first set forth in definite terms by one of the 
most famous of the astronomers of antiquity, Claudius 
Ptolemaeus Pelusinensis (100-170 A.D.), better known 
as Ptolemy of Alexandria. 

In his system the Earth occupied the centre ; 
while around it circled in order outwards the Moon, 
the planets Mercury and Venus, the Sun, and then 
the planets Mars, Jupiter, and Saturn. Beyond 



The Ancient View 

these again revolved the background of the heaven, 
upon which it was beHeved that the stars were fixed — 

" Stellis ardentibus aptum," 

as Virgil puts it (see Fig. i). 




Fig. I. — The Ptolemaic idea of the Universe. 

The Ptolemaic system persisted unshaken for about 
fourteen hundred years after the death of its author. 
Clearly men were flattered by the notion that their 
earth was the most important body in nature, that it 
stood still at the centre of the universe, and was the 
pivot upon which all things revolved. 

19 



CHAPTER II 

THE MODERN VIEW 

It is still well under four hundred years since the 
modern, or Copernican, theory of the universe sup- 
planted the Ptolemaic, which had held sway during 
so many centuries. In this new theory, propounded 
towards the middle of the sixteenth century by 
Nicholas Copernicus (1473-1543), a Prussian astro- 
nomer, the earth was dethroned from its central 
position and considered merely as one of a number 
of planetary bodies which revolve around the sun. 
As it is not a part of our purpose to follow in detail 
the history of the science, it seems advisable to begin 
by stating in a broad fashion the conception of the 
universe as accepted and believed in to-day. 

The Sun, the most important of the celestial bodies 
so far as we are concerned, occupies the central 
position ; not, however, in the whole universe, but 
only in that limited portion which is known as the 
Solar System. Around it, in the following order 
outwards, circle the planets Mercury, Venus, the 
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune 
(see Fig. 2, p. 21). At an immense distance beyond 
the solar system, and scattered irregularly through 
the depth of space, lie the stars. The two first- 
mentioned members of the solar system. Mercury 
and Venus, are known as the Inferior Planets ; 

20 



The Modern View 









Orbit of Nepfune 








^-' y 




^-^.^.^^^ 




^ 

y 






^\^ 


/" 






\, 








Orbit of I'rxxnu^ 








y^ 


Orbit of Safi,^^ 


\ 






[ 


M-^C. '" "T!,r?un 
\EarHi .."-■- Orbit/ 

\ Orbit C.; "fMf.rW / 


/ 






\ 


~ 


^--^^ 


/ 



Fig. 2. — The Copernican theory of the Solar System. 



21 



Astronomy of To-day 

and in their courses about the sun, they always 
keep well inside the path along which our earth 
moves. The remaining members (exclusive of the 
earth) are called Superior Planets, and their paths 
lie all outside that of the earth. 

The five planets, Mercury, Venus, Mars, Jupiter, 
and Saturn, have been known from all antiquity. 
Nothing then can bring home to us more strongly 
the immense advance which has taken place in 
astronomy during modern times than the fact that 
it is only 127 years since observation of the skies 
first added a planet to that time-honoured number. 
It was indeed on the 13th of March 1781, while en- 
gaged in observing the constellation of the Twins, 
that the justly famous Sir WilHam Herschel caught 
sight of an object which he did not recognise as 
having met with before. He at first took it for a 
comet, but observations of its movements during a 
few days showed it to be a planet. This body, which 
the power of the telescope alone had thus shown 
to belong to the solar family, has since become 
known to science under the name of Uranus. By 
its discovery the hitherto accepted hmits of the 
solar system were at once pushed out to twice their 
former extent, and the hope naturally arose that 
other planets would quickly reveal themselves in the 
immensities beyond. 

For a number of years prior to Herschel's great 
discovery, it had been noticed that the distances at 
which the then known planets circulated appeared 
to be arranged in a somewhat orderly progression 
outwards from the sun. This seeming plan, known 
to astronomers by the name of Bode's Law, was 

22 



The Modern View 

closely confirmed by the distance of the new planet 
Uranus. There still lay, however, a broad gap 
between the planets Mars and Jupiter. Had another 
planet indeed circuited there, the solar system would 
have presented an appearance of almost perfect 
order. But the void between Mars and Jupiter was 
unfilled ; the space in which one would reasonably 
expect to find another planet circHng was unaccount- 
ably empty. 

On the first day of the nineteenth century the 
mystery was however explained, a body being dis- 
covered 1 which revolved in the space that had 
hitherto been considered planetless. But it was a 
tiny globe hardly worthy of the name of planet. In 
the following year a second body was discovered re- 
volving in the same space ; but it was even smaller 
in size than the first. During the ensuing five years 
two more of these little planets were discovered. Then 
came a pause, no more such bodies being added to 
the system until half-way through the century, when 
suddenly the discovery of these so-called ^' minor 
planets " began anew. Since then additions to this 
portion of our system have rained thick and fast. The 
small bodies have received the name of Asteroids or 
Planetoids ; and up to the present time some six 
hundred of them are known to exist, all revolving 
in the previously unfilled space between Mars and 
Jupiter. 

In the year 1846 the outer boundary of the solar 
system was again extended by the discovery that a 
great planet circulated beyond Uranus. The new 
body, which received the name of Neptune, was 

* By the Italian astronomer, Piazzi, at Palermo. 
23 



Astronomy of To-day 

brought to light as the result of calculations made 
at the same time, though quite independently, by the 
Cambridge mathematician Adams, and the French 
astronomer Le Verrier. The discovery of Neptune 
differed, however, from that of Uranus in the following 
respect. Uranus was found merely in the course of 
ordinary telescopic survey of the heavens. The 
position of Neptune, on the other hand, was predicted 
as the result of rigorous mathematical investigations 
undertaken with the object of fixing the position of 
an unseen and still more distant body, the attraction 
of which, in passing by, was disturbing the position 
of Uranus in its revolution around the sun. Adams 
actually completed his investigation first ; but a delay 
at Cambridge in examining that portion of the sky, 
w^here he announced that the body ought just then 
to be, allowed France to snatch the honour of dis- 
covery, and the new planet was found by the observer 
Galle at Berlin, very near the place in the heavens 
which Le Verrier had mathematically predicted for it. 

Nearly fifty years later, that is to say, in our own 
time, another important planetary discovery was made. 
One of the recent additions to the numerous and con- 
stantly increasing family of the asteroids, a tiny body 
brought to light in 1898, turned out after all not to 
be circulating in the customary space between Mars 
and Jupiter, but actually in that betw^een our earth 
and Mars. This body is very small, not more than 
about tw^enty miles across. It has received the name 
of Eros (the Greek equivalent for Cupid), in allusion 
to its insignificant size as compared with the other 
leading members of the system. 

This completes the list of the planets which, so 

24 



The Modern View 

far, have revealed themselves to us. Whether others 
exist time alone will show. Two or three have been 
suspected to revolve beyond the path of Neptune ; 
and it has even been asserted, on more than one 
occasion, that a planet circulates nearer to the sun 
than Mercury. This supposed body, to which the 
name of '' Vulcan " was provisionally given, is said to 
have been 'discovered" in 1859 by a French doctor 
named Lescarbault, of Orgeres near Orleans ; but up 
to the present there has been no sufficient evidence 
of its existence. The reason why such uncertainty 
should exist upon this point is easy enough to 
understand, when we consider the overpowering glare 
which fills our atmosphere all around the sun's place 
in the sky. Mercury, the nearest known planet to 
the sun, is for this reason always very difficult to see ; 
and even when, in its course, it gets sufficiently far 
from the sun to be left for a short time above the 
horizon after sunset, it is by no means an easy object 
to observe on account of the mists which usually 
hang about low down near the earth. One oppor- 
tunity, however, offers itself from time to time to solve 
the riddle of an 'Mntra-Mercurial" planet, that is to 
say, of a planet which circulates within the path 
followed by Mercury. The opportunity in question is 
furnished by a total eclipse of the sun ; for when, 
during an eclipse of that kind, the body of the moon 
for a few minutes entirely hides the sun's face, and 
the dazzling glare is thus completely cut off, as- 
tronomers are enabled to give an unimpeded, though 
all too hurried, search to the region close around. A 
goodly number of total eclipses of the sun have, how- 
ever, come and gone since the days of Lescarbault, 

25 



Astronomy of To-day 

and no planet, so far, has revealed itself in the intra- 
Mercurial space. It seems, therefore, quite safe to 
affirm that no globe of sufficient size to be seen by 
means of our modern telescopes circulates nearer to 
the sun than the planet Mercury. 

Next in importance to the planets, as permanent 
members of the solar system, come the relatively 
small and secondary bodies known by the name of 
Satellites. The name satellite is derived from a Latin 
word signifying an attendant ; for the bodies so-called 
move along always in close proximity to their re- 
spective ^' primaries," as the planets which they 
accompany are technically termed. 

The satellites cannot be considered as allotted with 
any particular regularity among the various members 
of the system ; several of the planets, for instance, 
having a goodly number of these bodies accompany- 
ing them, while others have but one or two, and 
some again have none at all. Taking the planets in 
their order of distance outward from the Sun, we find 
that neither Mercury nor Venus are provided with 
satellites ; the Earth has only one, viz. our neighbour 
the Moon ; while Mars has but two tiny ones, so 
small indeed that one might imagine them to be 
merely asteroids, which had wandered out of their 
proper region and attached themselves to that planet. 
For the rest, so far as we at present know, Jupiter 
possesses seven,^ Saturn ten, Uranus four, and Neptune 
one. It is indeed possible, nay more, it is extremely 
probable, that the two last-named planets have a 
greater number of these secondary bodies revolving 
around them ; but, unfortunately, the Uranian and 

^ Probably eight. (See note, page 232.) 
26 



The Modern View 

Neptunian systems are at such immense distances from 
us, that even the magnificent telescopes of to-day can 
extract very little information concerning them. 

From the distribution of the satellites, the reader 
will notice that the planets relatively near to the 
sun are provided with few or none, while the more 
distant planets are richly endowed. The conclusion, 
therefore, seems to be that nearness to the sun is in 
some way unfavourable either to the production, or to 
the continued existence, of satellites. 

A planet and its satelHtes form a repetition of the 
solar system on a tiny scale. Just as the planets re- 
volve around the sun, so do these secondary bodies 
revolve around their primaries. When Galileo, in 
1610, turned his newly invented telescope upon 
Jupiter, he quickly recognised in the four circling 
moons which met his gaze, a miniature edition of the 
solar system. 

Besides the planets and their satellites, there are 
two other classes of bodies which claim membership 
of the solar system. These are Comets and Meteors. 
Comets differ from the bodies which we have just 
been describing in that they appear filmy and trans- 
parent, whereas the others are solid and opaque. 
Again, the paths of the planets around the sun and of 
the satellites around their primaries are not actually 
circles ; they are ovals, but their ovalness is not of a 
marked degree. The paths of comets on the other 
hand are usually very oval ; so that in their courses 
many of them pass out as far as the known limits of 
the solar system, and even far beyond. It should be 
mentioned that nowadays the tendency is to consider 
comets as perm.anent members of the system, though 

27 



Astronomy of To-day 

this was formerly not by any means an article of 
faith with astronomers. 

Meteors are very small bodies, as a rule perhaps no 
larger than pebbles, which move about unseen in 
space, and of which we do not become aware until 
they arrive very close to the earth. They are then 
made visible to us for a moment or two in conse- 
quence of being heated to a white heat by the friction 
of rushing through the atmosphere, and are thus 
usually turned into ashes and vapour long before 
they reach the surface of our globe. Though occa- 
sionally a meteoric body survives the fiery ordeal, and 
reaches the earth more or less in a solid state to bury 
itself deep in the soil, the majority of these celestial 
visitants constitute no source of danger whatever for 
us. Any one who will take the trouble to gaze at the 
sky for a short time on a clear night, is fairly certain 
to be rewarded with the view of a meteor. The 
impression received is as if one of the stars had 
suddenly left its accustomed place, and dashed across 
the heavens, leaving in its course a trail of light. It 
is for this reason that meteors are popularly known 
under the name of ''shooting stars." 



CHAPTER III 

THE SOLAR SYSTEM 

We have seen, in the course of the last chapter, that 
the solar system is composed as follows : — there is 
a central body, the sun, around which revolve along 
stated paths a number of important bodies known as 
planets. Certain of these planets, in their courses, 
carry along in company still smaller bodies called 
satellites, which revolve around them. With regard, 
however, to the remaining members of the system, 
viz. the comets and the meteors, it is not advisable at 
this stage to add more to what has been said in the 
preceding chapter. For the time being, therefore, we 
will devote our attention merely to the sun, the 
planets, and the satellites. 

Of what shape then are these bodies ? Of one 
shape, and that one alone which appears to char- 
acterise all solid objects in the celestial spaces : they 
are spherical, which means round like a ball. 

Each of these spherical bodies rotates ; that is to 
say, turns round and round, as a top does when it is 
spinning. This rotation is said to take place ^'upon 
an axis," a statement which may be explained as 
follows : — Imagine a ball with a knitting-needle run 
right through its centre. Then imagine this needle 
held pointing in one fixed direction while the ball 
is turned round and round. Well, it is the same 

29 



Astronomy of To-day 

thing with the earth. As it journeys about the sun, 
it keeps turning round and round continually as if 
pivoted upon a mighty knitting needle transfixing 
it fron^i North Pole to South Pole. In reality, how- 
ever, there is no such material axis to regulate the 
constant direction of the rotation, just as there are 
no actual supports to uphold the earth itself in space. 
The causes which keep the celestial spheres poised, 
and which control their motions, are far more wonder- 
ful than any mechanical device. 

At this juncture it will be well to emphasise the 
sharp distinction between the terms rotation and re- 
volution. The term *^ rotation" is invariably used by 
astronomers to signify the motion which a celestial 
body has upon an axis; the term ^^ revolution," on 
the other hand, is used for the movement of one 
celestial body around another. Speaking of the earth, 
for instance, we say, that it rotates on its axis, and 
that it revolves around the sun. 

So far, nothing has been said about the sizes of the 
members of our system. Is there any stock size, any 
pattern according to which they may be judged ? 
None whatever ! They vary enormously. Very much 
the largest of all is the Sun, which is several hundred 
times larger than all the planets and satellites of 
the system rolled together. Next comes Jupiter and 
afterwards the other planets in the following order of 
size : — Saturn, Uranus, Neptune, the Earth, Venus, 
Mars, and Mercury. Very much smaller than any of 
these are the asteroids, of which Ceres, the largest, 
is less than 500 miles in diameter. It is, by the way, 
a strange fact that the zone of asteroids should mark 
the separation of the small planets from the giant 

30 



The Solar System 



ones. The following table, giving roughly the various 
diameters of the sun and the principal planets in 
miles, will clearly illustrate the great discrepancy in 
size which prevails in the system. 



Sun . 


. 866,540 miles 


Mercury . 


2,765 „ 


Venus 


. . 7,826 „ 


Earth 


7,918 „ 


Mars 


4,332 „ 


ZONE O 


F ASTEROIDS 


Jupiter 


• 87,380 „ 


Saturn 


• 73,125 „ 


Uranus 1 . 


34,900 „ 


Neptune 1. 


32,900 „ 



It does not seem possible to arrive at any general-, 
isation from the above data, except it be to state that 
there is a continuous increase in size from Mercury to 
the earth, and a similar decrease in size from Jupiter 
outwards. Were Mars greater than the earth, the 
planets could then with truth be said to increase in 
size up to Jupiter, and then to decrease. But the 
zone of asteroids, and the relative smallness of Mars, 
negative any attempt to regard the dimensions of the 
planets as an orderly sequence. 

Next with respect to relative distance from the sun, 
Venus circulates nearly twice as far from it as 
Mercury, the earth nearly three times as far, and 

•^ As there seems to be much difference of opinion concerning the dia- 
meters of Uranus and Neptune, it should here be mentioned that the above 
figures are taken from Professor F. R. Moulton's Introduction to Astro- 
no?ny (1906). They are there stated to be given on the authority of 
" Barnard's many measures at the Lick Observatory." 

31 



Astronomy of To-day 

Mars nearly four times. After this, just as we found 
a sudden increase in size, so do we meet with a 
sudden increase in distance. Jupiter, for instance, is 
about thirteen times as far as Mercury, Saturn about 
twenty-five times, Uranus about forty-nine times, and 
Neptune about seventy-seven. (See Fig. 2, p. 21.) 

It will thus be seen how enormously the solar 
system was enlarged in extent by the discovery of the 
outermost planets. The finding of Uranus plainly 
doubled its breadth ; the finding of Neptune made it 
more than half as broad again. Nothing indeed can 
better show the import of these great discoveries than 
to take a pair of compasses and roughly set out the 
above relative paths in a series of concentric circles 
upon a large sheet of paper, and then to consider 
that the path of Saturn was the supposed boundary 
of our solar system prior to the year 1781. 

We have seen that the usual shape of celestial 
bodies themselves is spherical. Of what form then 
are their paths, or oj^bitSy as these are called ? One 
might be inclined at a venture to answer ^^ circular," 
but this is not the case. The orbits of the planets 
cannot be regarded as true circles. They are ovals, 
or, to speak in technical language, '' ellipses." Their 
ovalness or '^ ellipticity " is, however, in each case not 
by any means of the same degree. Some orbits — for 
instance, that of the earth — differ only slightly from 
circles ; while others — those of Mars or Mercury, for 
example — are markedly elliptic. The orbit of the tiny 
planet Eros is, however, far and away the most elliptic 
of all, as we shall see when we come to deal with 
that little planet in detail. 

It has been stated that the sun and planets are 

32 



The Solar System 



always rotating. The various rates at which they do 
so will, however, be best appreciated by a comparison 
with the rate at which the earth itself rotates. 

But first of all, let us see what ground we have, if 
any, for asserting that the earth rotates at all ? 

If we carefully watch the heavens we notice that 
the background of the sky, wath all the brilliant objects 
which sparkle in it, appears to turn once round us in 
about twenty-four hours ; and that the pivot upon 
which this movement takes place is situated some- 
where near what is known to us as the Pole Star. 
This was one of the earliest facts noted with regard 
to the sky ; and to the men of old it therefore seems 
as if the heavens and all therein were always 
revolving around the earth. It w^as natural enough 
for them to take this view, for they had not the 
slightest idea of the immense distance of the celestial 
bodies, and in the absence of any knowledge of the 
kind they were inclined to imagine them compara- 
tively near. It was indeed only after the lapse of 
many centuries, when men had at last realised the 
enormous gulf which separated them from even 
the nearest object in the sky, that a more reason- 
able opinion began to prevail. It was then seen 
that this revolution of the heavens about the earth 
could be more easily and more satisfactorily ex- 
plained by supposing a mere rotation of the solid 
earth about a fixed axis, pointed in the direction of 
the polar star. The probability of such a rotation 
on the part of the earth itself was further strengthened 
by the observations made with the telescope. When 
the surfaces of the sun and planets were carefully 
studied these bodies were seen to be rotating. This 

33 C 



Astronomy of To-day 

being the case, there could not surely be much hesi- 
tation in granting that the earth rotated also ; par- 
ticularly when it so simply explained the daily 
movement of the sky, and saved men from the 
almost inconceivable notion that the whole stupen- 
dous vaulted heaven was turning about them once 
in every twenty-four hours. 

If the sun be regularly observed through a tele- 
scope, it will gradually be gathered from the slow 
displacement of sunspots across its face, their dis- 
appearance at one edge and their reappearance again 
at the other edge, that it is rotating on an axis in a 
period of about twenty-six days. The movement, 
too, of various well-known markings on the surfaces 
of the planets Mars, Jupiter, and Saturn proves to 
us that these bodies are rotating in periods, which 
are about twenty-four hours for the first, and about 
ten hours for each of the other two. With regard, 
however, to Uranus and Neptune there is much more 
uncertainty, as these planets are at such great dis- 
tances that even our best telescopes give but a 
confused view of the markings which they display ; 
still a period of rotation of from ten to twelve hours 
appears to be accepted for them. On the other hand 
the constant blaze of sunlight in the neighbourhood 
of Mercury and Venus equally hampers astronomers 
in this quest. The older telescopic observers con- 
sidered that the rotation periods of these two planets 
were about the same as that of the earth ; but of recent 
years the opinion has been gaining ground that they 
turn round on their axes in exactly the same time as 
they revolve about the sun. This question is, how- 
ever, a very doubtful one, and will be again referred 

34 



The Solar System 



to later on ; but, putting it on one side, it will be seen 
from what we have said above, that the rotation 
periods of the other planets of our system are usually 
about twenty-four hours, or under. The fact that 
the rotation period of the sun should run into days 
need not seem extraordinary when one considers its 
enormous size. 

The periods taken by the various planets to revolve 
around the sun is the next point which has to be 
considered. Here, too, it is well to start with the 
earth's period of revolution as the standard, and to 
see how the periods taken by the other planets com- 
pare with it. 

The earth takes about 365^ days to revolve around 
the sun. This period of time is known to us as a 
''year." The following table shows in days and years 
the periods taken by each of the other planets to 
make a complete revolution round the sun ; — 



Mercury 


about 


88 days. 


Venus 




226 „ 


Mars 




I year and 321 days. 


Jupiter 




IT years and 313 days. 


Saturn 




29 years and 167 days. 


Uranus 




84 years and 7 days. 


Neptune 




164 years and 284 days. 



From these periods we gather an important fact, 
namely, that the nearer a planet is to the sun the 
faster it revolves. 

Compared with one of our years what a long time 
does an Uranian, or Neptunian, ''year" seem? For 
instance, if a " year " had commenced in Neptune about 
the middle of the reign of George II., that "year" 

35 



Astronomy of To-day 

would be only just coming to a close ; for the planet 
is but now arriving back to the position, with regard 
to the sun, which it then occupied. Uranus, too, has 
only completed a little more than ij of its ^'years'' 
since Herschel discovered it. 

Having accepted the fact that the planets are 
revolving around the sun, the next point to be in- 
quired into is : — What are the positions of their orbits, 
or paths, relatively to each other ? 

Suppose, for instance, the various planetary orbits 
to be represented by a set of hoops of different sizes, 
placed one within the other, and the sun by a small 
ball in the middle of the whole ; in what positions 
will these hoops have to be arranged so as to imitate 
exactly the true condition of things ? 

First of all let us suppose the entire arrangement, 
ball and hoops, to be on one level, so to speak. This 
may be easily compassed by imagining the hoops as 
floating, one surrounding the other, with the ball in 
the middle of all, upon the surface of still water. 
Such a set of objects would be described in astrono- 
mical parlance as being in the same plane. Suppose, 
on the other hand, that some of these floating hoops 
are tilted with regard to the others, so that one half of 
a hoop rises out of the water and the other half conse- 
quently sinks beneath the surface. This indeed is 
the actual case with regard to the planetary orbits. 
They do not by any means He all exactly in the same 
plane. Each one of them is tilted, or inclined, a little 
with respect to the plane of the earth's orbit, which 
astronomers, for convenience, regard as the level oi 
the solar system. This tilting, or '^ inclination," is, in 
the larger planets, greatest for the orbit of Mercury, 

36 



The Solar System 



least for that of Uranus. Mercury's orbit is inclined to 
that of the earth at an angle of about 7°, that of Venus 
at a little over 3°, that of Saturn 24° ; while in those of 
Mars, Neptune, and Jupiter the inclination is less than 
2^ But greater than any of these is the inclination 
of the orbit of the tiny planet Eros, viz. nearly 11°. 

The systems of satellites revolving around their 
respective planets being, as we have already pointed 
out, mere miniature editions of the solar system, the 
considerations so far detailed, which regulate the 
behaviour of the planets in their relations to the sun, 
will of necessity apply to the satellites very closely. 
In one respect, however, a system of satellites differs 
materially from a system of planets. The central body 
around which planets are in motion is self-luminous, 
whereas the planetary body around which a satellite 
revolves is not. True, planets shine, and shine very 
brightly too ; as, for instance, Venus and Jupiter. But 
they do not give forth any light of their own, as the 
sun does ; they merely reflect the sunlight which they 
receive from him. Putting this one fact aside, the 
analogy between the planetary system and a satellite 
system is remarkable. The satellites are spherical in 
form, and differ markedly in size ; they rotate, so far 
as we know, upon their axes in varying times ; they re- 
volve around their governing planets in orbits, not 
circular, but elliptic ; and these orbits, furthermore, do 
not of necessity lie in the same plane. Last of all the 
satelHtes revolve around their primaries at rates which 
are directly comparable with those at which the planets 
revolve around the sun, the rule in fact holding good 
that the nearer a satellite is to its primary the faster it 
revolves. 

37 



CHAPTER IV 

CELESTIAL MECHANISM 

As soon as we begin to inquire closely into the actual 
condition of the various members of the solar system 
we are struck with a certain distinction. We find that 
there are two quite different points of view from w^hich 
these bodies can be regarded. For instance^ w^e may 
make our estimates of them either as regards volume 
— that is to say, the mere room w^hich they take up ; or 
as regards mass — that is to say, the amount of matter 
w^hich they contain. 

Let us imagine two globes of equal volume ; in 
other words, w'hich take up an equal amount of space. 
One of these globes, however, may be composed of 
material much more tightly put together than in the 
other ; or of greater density^ as the term goes. That 
globe is said to be the greater of the two in mass. 
Were such a pair of globes to be weighed in scales, 
one globe in each pan, we should see at once, by its 
weighing down the other, w^hich of the two was com- 
posed of the more tightly packed materials ; and we 
should, in astronomical parlance, say of this one that 
it had the greater mass. 

Volume being merely another word for size, the 
order of the members of the solar system, with 
regard to their volumes, will be as follows, begin- 
ning with the greatest : — the Sun, Jupiter, Saturn, 

38 



Celestial Mechanism 

Uranus, Neptune, the Earth, Venus, Mars, and 
Mercury. 

With regard to mass the same order strangely 
enough holds good. The actual densities of the 
bodies in question are, however, very different. The 
densest or closest packed body of all is the Earth, 
which is about five and a half times as dense as if 
it were composed entirely of water. Venus follows 
next, then Mars, and then Mercury. The remaining 
bodies, on the other hand, are relatively loose in struc- 
ture. Saturn is the least dense of all, less so than 
water. The density of the Sun is a little greater than 
that of water. 

This method of estimating is, however, subject to a 
qualification. It must be remembered that in speaking 
of the Sun, for instance, as being only a little denser 
than water, we are merely treating the question from 
the point of view of an average. Certain parts of it in 
fact will be ever so much denser than water : those 
are the parts in the centre. Other portions, for 
instance, the outside portions, will be very much less 
dense. It will easily be understood that in all such 
bodies the densest or most compressed portions are 
to be found towards the centre ; while the portions 
towards the exterior being less pressed upon, will be 
less dense. 

We now reach a very important point, the question 
of Gravitation. d'avitatioUy or gravity , as it is often 
called, is the attractive force which, for instance, 
causes objects to fall to the earth. Now it seems 
rather strange that one should say that it is owing to 
a certain force that things fall towards the earth. All 
things seem to us to fall so of their own accord, as if it 

39 



Astronomy of To-day 

were quite natural, or rather most unnatural if they did 
not. Why then require a '^ force" to make them fall ? 

The story goes that the great Sir Isaac Newton was 
set a-thinking on this subject by seeing an apple fall 
from a tree to the earth. He then carried the train of 
thought further ; and, by studying the movements of 
the moon, he reached the conclusion that a body even 
so far off as our satellite would be drawn towards the 
earth in the same manner. This being the case, one 
will naturally ask why the moon herself does not fall 
in upon the earth. The answer is indeed found to 
be that the moon is travelling round and round the 
earth at a certain rapid pace, and it is this very samxC 
rapid pace which keeps her from falling in upon us. 
Any one can test this simple fact for himself. If we 
tie a stone to the end of a string, and keep whirling it 
round and round fast enough, there will be a strong 
pull from the stone in an outward direction, and the 
string will remain tight all the time that the stone is 
being whirled. If, however, we gradually slacken the 
speed at which we are making the stone whirl, a 
moment will come at length when the string will 
become limp, and the stone will fall back towards our 
hand. 

It seems, therefore, that there are two causes which 
maintain the stone at a regular distance all the time it 
is being steadily whirled. One of these is the con- 
tinual pull inward towards our hand by means of the 
string. The other is the continual pull away from us 
caused by the rate at which the stone is travelling. 
When the rate of whirling is so regulated that these 
pulls exactly balance each other, the stone travels com- 
fortably round and round, and shows no tendency 

40 



Celestial Mechanism 

either to fall back upon our hand or to break the 
string and fly away into the air. It is indeed pre- 
cisely similar with regard to the moon. The continual 
pull of the earth's gravitation takes the place of the 
string. If the moon were to go round and round 
slower than it does, it would tend to fall in towards 
the earth ; if, on the other hand, it were to go faster, 
it would tend to rush away into space. 

The same kind of pull which the earth exerts 
upon the objects at its surface, or upon its satellite, 
the moon, exists through space so far as we know. 
"Every particle of matter in the universe is found in 
fact to attract every other particle. The moon, for 
instance, attracts the earth also, but the controlling 
force is on the side of the much greater mass of the 
earth. This force of gravity or attraction of gravita- 
tion, as it is also called, is perfectly regular in its 
action. Its power depends first of all exactly upon 
the mass of the body which exerts it. The gravita- 
tional pull of the sun, for instance, reaches out to 
an enormous distance, controlling perhaps, in their 
courses, unseen planets circling far beyond the orbit 
of Neptune. Again, the strength with which the 
force of gravity acts depends upon distance in a 
regularly diminishing proportion. Thus, the nearer 
an object is to the earth, for instance, the stronger 
is the gravitational pull which it gets from it ; the 
farther off it is, the weaker is this pull. If then the 
moon were to be brought nearer to the earth, the 
gravitational pull of the latter would become so 
much stronger that the moon's rate of motion would 
have also to increase in due proportion to prevent 
her from being drawn into the earth. Last of all, 

41 



Astronomy of To-day 

the point in a body from which the attraction of 
gravitation acts, is not necessarily the centre of the 
body, but rather what is known as its centre of 
gravity, that is to say, the balancing point of all the 
matter which the body contains. 

It should here be noted that the moon does not 
actually revolve around the centre of gravity of the 
earth. What really happens is that both orbs re- 
volve around their common centre of gravity, which 
is a point within the body of the earth, and situ- 
ated about three thousand miles from its centre. 
In the same manner the planets and the sun revolve 
around the centre of gravity of the solar system, 
which is a point within the body of the sun. 

The neatly poised movements of the planets around 
the sun, and of the satellites around their respective 
planets, will therefore be readily understood to result 
from a nice balance between gravitation and speed 
of motion. 

The mass of the earth is ascertained to be about 
eighty times that of the moon. Our knowledge of 
the mass of a planet is learned from comparing the 
revolutions of its satellite or satellites around it, with 
those of the moon around the earth. We are thus 
enabled to deduce what the mass of such a planet 
would be compared to the earth's mass ; that is to 
say, a study, for instance, of Jupiter's satellite system 
shows that Jupiter must have a mass nearly three 
hundred and eighteen times that of our earth. In 
the same manner we can argue out the mass of 
the sun from the movements of the planets and 
other bodies of the system around it. With regard, 
however, to Venus and Mercury, the problem is by 

42 



Celestial Mechanism 

no means such an eas}^ one, as these bodies have no 
satelHtes. For information in this latter case we have 
to rely upon such uncertain evidence as, for instance, 
the sHght disturbances caused in the motion of the 
earth by the attraction of these planets when they 
pass closest to us, or their observed effect upon the 
motions of such comets as may happen to pass near 
to them. 

Mass and weight, though often spoken of as one 
and the same thing, are by no means so. Mass, as 
we have seen, merely means the amount of matter 
which a body contains. The weight of a body, on 
the other hand, depends entirely upon the gravitational 
pull which it receives. The force of gravity at the 
surface of the earth is, for instance, about six times as 
great as that at the surface of the moon. All bodies, 
therefore, weigh about six times as much on the earth 
as they would upon the moon ; or, rather, a body 
transferred to the moon's surface would weigh only 
about one-sixth of what it did on the terrestrial surface. 
It will therefore be seen that if a body of given mass 
were to be placed upon planet after planet in turn, its 
weight would regularly alter according to the force of 
gravity at each planet's surface. 

Gravitation is indeed one of the greatest mysteries 
of nature. What it is, the means by which it acts, or 
why such a force should exist at all, are questions to 
which so far we have not had even the merest hint of 
an answer. Its action across space appears to be 
instantaneous. 

The intensity of gravitation is said in mathematical 
parlance ^^to vary inversely with the square of the 
distance." This means that at twice the distance the 

43 



Astronomy of To-day 

pull will become only one-quarter as strong, and not 
one-half as otherwise might be expected. At four 
times the distance, therefore; it will be one-sixteenth as 
strong. At the earth's surface a body is pulled by the 
earth's gravitation, or ''falls," as we ordinarily term it, 
through 1 6 feet in one second of time ; whereas at the 
distance of the moon the attraction of the earth is so 
very much weakened that a body would take as long 
as one minute to fall through the same space. 

Newton's investigations showed that if a body were 
to be placed at rest in space entirely away from the 
attraction of any other body it would remain always 
in a motionless condition, because there would plainly 
be no reason why it should move in any one direc- 
tion rather than in another. And, similarly, if a body 
were to be projected in a certain direction and at 
a certain speed, it would move always in the same 
direction and at the same speed so long as it did 
not come within the gravitational attraction of any 
other body. 

The possibiHty of an interaction between the celestial 
orbs had occurred to astronomers before the time of 
Newton ; for instance, in the ninth century to the 
Arabian Musa-ben-Shakir, to Camillus Agrippa in 
1553, and to Kepler, who suspected its existence from 
observation of the tides. Horrox also, writing in 1635, 
spoke of the moon as moved by an emanation from 
the earth. But no one prior to Newton attempted 
to examine the question from a mathematical stand- 
point. 

Notwithstanding the acknowledged truth and far- 
reaching scope of the law of gravitation — for we find 
its effects exemplified in every portion of the universe — 

44 



Celestial Mechanism 

there are yet some minor movements which it does 
not account for. For instance, there are small irre- 
gularities in the movement of Mercury which cannot 
be explained by the influence of possible intra-Mer- 
curial planets, and similarly there are slight unaccount- 
able deviations in the motions of our neighbour the 
Moon. 



45 



CHAPTER V 

CELESTIAL DISTANCES 

Up to this we have merely taken a general view 
of the solar system — a bird's-eye view, so to speak, 
from space. 

In the course of our inquiry we noted in a rough 
way the relative distances at which the various planets 
move around the sun. But we have not yet stated 
what these distances actually are, and it were there- 
fore well now to turn our attention to this impor- 
tant matter. 

Each of us has a fair idea of what a mile is. It 
is a quarter of an hour's sharp walk, for instance ; 
or yonder village or building, we know, lies such 
and such a number of miles away. 

The measurements which have already been given 
of the diameters of the various bodies of the solar 
system appear very great to us, who find that a 
walk of a few miles at a time taxes our strength ; 
but they are a mere nothing wheri we consider the 
distances from the sun at which the various planets 
revolve in their orbits. 

The following table gives these distances in round 
numbers. As here stated they are what are called 
''mean" distances; for, as the orbits are oval, the 
planets vary in their distances from the sun, and 

46 



Celestial Distances 

we are therefore obliged to strike a kind of average 
for each case : — 

miles. 



Mercury 


about 


36,000,000 


Venus 


}5 


67,200,000 


Earth 


55 


92,900,000 


Mars 


55 


141,500,000 


Jupiter 


5J 


483,300,000 


Saturn 


55 


886,000,000 


Uranus 


55 


1,781,900,000 


Neptune 


J) 


2,791,600,000 



From the above it will be seen at a glance that 
we have entered upon a still greater scale of distance 
than in dealing with the diameters of the various 
bodies of the system. In that case the distances 
were limited to thousands of miles ; in this, however, 
we have to deal with millions. A million being 
ten hundred thousand, it will be noticed that even 
the diameter of the huge sun is well under a million 
miles. 

How indeed are we to get a grasp of such 
distances, when those to which we are ordinarily 
accustomed — the few miles' walk, the little stretch 
of sea or land which we gaze upon around us — are 
so utterly minute in comparison ? The fact is, that 
though men may think that they can picture in their 
minds such immense distances, they actually can 
not. In matters like these we unconsciously employ 
a kind of convention, and we estimate a thing as 
being two or three or more times the size of another. 
More than this we are unable to do. For instance, 
our ordinary experience of a mile enables us to 
judge, in a way, of a stretch of several miles, such 

47 



Astronomy of To-day 

as one can take in with a glance ; but in our estima- 
tion of a thousand miles, or even of one hundred, 
we are driven back upon a mental trick, so to 
speak. 

In our attempts to realise such immense distances 
as those in the solar system we are obliged to have 
recourse to analogies ; to comparisons with other and 
simpler facts, though this is at the best a mere self- 
cheating device. The analogy which seems most 
suited to our purpose here, and one which has often 
been employed by writers, is borrowed from the rate 
at which an express train travels. 

Let us imagine, for instance, that we possess an 
express train which is capable of running anywhere, 
never stops, never requires fuel, and always goes along 
at sixty miles an hour. Suppose we commence by 
employing it to gauge the size of our own planet, the 
earth. Let us send it on a trip around the equator, 
the span of which is about 24,000 miles. At its sixty- 
miles-an-hour rate of going, this journey will take 
nearly 17 days. Next let us send it from the earth lo 
the moon. This distance, 240,000 miles, being ten 
times as great as the last, will of course take ten times 
as long to cover, namely, 170 days ; that is to say, 
nearly half a year. Again, let us send it still further 
afield, to the sun, for example. Here, however, it 
enters upon a journey which is not to be measured in 
thousands of miles, as the others were, but in millions. 
The distance from the earth to the sun, as we have 
seen in the foregoing table, is about 93 millions of 
miles. Our express train would take about i']^ years 
to traverse this. 

Having arrived at the sun, let us suppose that our 

48 



Celestial Distances 

train makes a tour right round it. This will take more 
than five years. 

Supposing, finally, that our train were started from 
the sun, and made to run straight out to the known 
boundaries of the solar system, that is to say, as far 
as the orbit of Neptune, it would take over 5000 years 
to traverse the distance. 

That sixty miles an hour is a very great speed any 
one, I think, will admit who has stood upon the plat- 
form of a country station while one of the great mail 
trains has dashed past. But are not the immensities 
of space appaUing to contemplate, when one realises 
that a body moving incessantly at such a rate would 
take so long as 10,000 years to traverse merely the 
breadth of our solar system ? Ten thousand years ! 
Just try to conceive it. Why, it is only a little more 
than half that time since the Pyramids were built, 
and they mark for us the Dawn of History. And 
since then half-a-dozen mighty empires have come 
and gone ! 

Having thus concluded our general survey of the 
appearance and dimensions of the solar system, let 
us next inquire into its position and size in relation 
to what we call the Universe. 

A mere glance at the night sky, when it is free from 
clouds, shows us that in every direction there are 
stars ; and this holds good, no matter what portion 
of the globe we visit. The same is really true of the 
sky by day, though in that case we cannot actually 
see the stars, for their light is quite overpowered by 
the dazzling light of the sun. 

We thus reach the conclusion that our earth, that 
our solar system in fact, lies plunged within the midst 

49 D 



Astronomy of To-day 

of a great tangle of stars. What position, by the way, 
do we occupy in this mighty maze ? Are we at the 
centre, or anywhere near the centre, or where ? 

It has been indeed amply proved by astronomical 
research that the stars are bodies giving off a light of 
their own, just as our sun does ; that they are in fact 
suns, and that our sun is merely one, perhaps indeed 
a very unimportant member, of this great universe 
of stars. Each of these stars, or suns, besides, may 
be the centre of a system similar to what we call our 
solar system, comprising planets and satellites, comets 
and meteors ; — or perchance indeed some further 
variety of attendant bodies of which we have no ex- 
ample in our tiny corner of space. But as to w^hether 
one is right in a conjecture of this kind, there is up 
to the present no proof whatever. No telescope has 
yet shown a planet in attendance upon one of these 
distant suns ; for such bodies, even if they do exist, 
are entirely out of the range of our mightiest instru- 
ments. On what then can we ground such an as- 
sumption ? Merely upon analogy ; upon the common- 
sense deduction that as the stars have characteristics 
similar to our particular star, the sun, it would seem 
unlikely that ours should be the only such body in 
the whole of space which is attended by a planetary 
system. 

^' The Stars/' using that expression in its most 
general sense, do not lie at one fixed distance from 
us, set here and there upon a background of sky. 
There is in fact no background at all. The brilliant 
orbs are all around us in space, at different distances 
from us and from each other ; and w^e can gaze 
between them out into the blackness of the void 

50 



Celestial Distances 

which, perhaps, continues to extend unceasingly long 
after the very outposts of the stellar universe has 
been left behind. Shall we then start our imaginary 
express train once more, and send it out towards the 
nearest of the stars ? This would, however, be a use- 
less experiment. Our express-train method of gauging 
space would fail miserably in the attempt to bring 
home to us the mighty gulf by which we are now 
faced. Let us therefore halt for a moment and look 
back upon the orders of distance with which we have 
been deahng. First of all we dealt with thousands of 
miles. Next we saw how they shrank into insignifi- 
cance when we embarked upon millions. We found, 
indeed, that our sixty-mile-an-hour train, rushing along 
without ceasing, would consume nearly the whole 
of historical time in a journey from the sun to 
Neptune. 

In the spaces beyond the solar system we are faced, 
however, by a new order of distance. From sun to 
planets is measured in millions of miles, but from sun 
to sun is measured in bilHons. But does the mere 
stating of this fact convey anything ? I fear not. For 
the word ^'billion" runs as glibly off the tongue as 
"million," and both are so wholly unrealisable by us 
that the actual difference between them might easily 
pass unnoticed. 

Let us, however, make a careful comparison. What 
is a million ? It is a thousand thousands. But what 
is a billion ? It is a million millions. Consider for 
a moment ! A million of millions. That means 
a million, each unit of which is again a million. 
In fact every separate ** i " in this million is itself 
a million. Here is a way of trying to realise this 

51 



Astronomy of To-day 

gigantic number. A million seconds make only 
eleven and a half days and nights. But a billion 
seconds will make actually more than thirty thousand 
years ! 

Having accepted this, let us try and probe with our 
express train even a little of the new gulf which now 
lies before us. At our old rate of going it took almost 
two years to cover a million miles. To cover a billion 
miles — that is to say, a million times this distance — 
would thus take of course nearly two million years. 
Alpha Centauri, the nearest star to our earth, is some 
twenty-five billions of miles away. Our express train 
would thus take about fifty millions of years to 
reach it I 

This shows how useless our illustration, appropriate 
though it seemed for interplanetary space, becomes 
when applied to the interstellar spaces. It merely 
gives us millions in return for billions ; and so the 
mind, driven in upon itself, whirls round and round 
Hke a squirrel in its revolving cage. There is, how- 
ever, a useful illustration still left us, and it is the one 
which astronomers usually employ in dealing with the 
distances of the stars. The illustration in question is 
taken from the velocity of light. 

Light travels at the tremendous speed of about 
186,000 miles a second. It therefore takes only about 
a second and a quarter to come to us from the moon. 
It traverses the 93,000,000 of miles which separate 
us from the sun in about eight minutes. It travels 
from the sun out to Neptune in about four hours, 
which means that it would cross the solar system 
from end to end in eight. To pass, however, across 
the distance which separates us from Alpha Centauri 

52 



Celestial Distances 

it would take so long as about four and a quarter 
years ! 

Astronomers, therefore, agree in estimating the 
distances of the stars from the point of view of the 
time which light would take to pass from them to our 
earth. They speak of that distance which light takes 
a year to traverse as a ^^ light year." According to 
this notation, Alpha Centauri is spoken of as being 
about four and a quarter light years distant from us. 

Now as the rays of light coming from Alpha Cen- 
tauri to us are chasing one another incessantly across 
the gulf of space, and as each ray left that star some 
four years before it reaches us, our view of the star 
itself must therefore be always some four years old. 
Were then this star to be suddenly removed from the 
universe at any moment, we should continue to see it 
still in its place in the sky for some four years more, 
after which it would suddenly disappear. The rays 
which had already started upon their journey towards 
our earth must indeed continue travelling, and reach- 
ing us in their turn until the last one had arrived ; 
after which no more would come. 

We have drawn attention to Alpha Centauri as the 
nearest of the stars. The majority of the others in- 
deed are ever so much farther. We can only hazard 
a guess at the time it takes for the rays from many of 
them to reach our globe. Suppose, for instance, we 
see a sudden change in the light of any of these re- 
mote stars, we are inclined to ask ourselves when that 
change did actually occur. Was it in the days of 
Queen Elizabeth, or at the time of the Norman Con- 
quest; or was it when Rome was at the height of her 
glory, or perhaps ages before that when the Pyramids 

53 



Astronomy of To-day 

of Egypt were being built ? Even the last of these 
suppositions cannot be treated lightly. We have 
indeed no real knowledge of the distance from us of 
those stars which our giant telescopes have brought 
into view out of the depths of the celestial spaces. 



54 



CHAPTER VI 

CELESTIAL MEASUREMENT 

Had the telescope never been invented our knowledge 
of astronomy would be trifling indeed. 

Prior to the year 1610, when Galileo first turned 
the new instrument upon the sky, all that men knew 
of the starry realms was gathered from observation 
with their own eyes unaided by any artificial means. 
In such researches they had been very much at a 
disadvantage. The sun and moon, in their opinion, 
were no doubt the largest bodies in the heavens, for 
the mere reason that they looked so ! The mighty 
solar disturbances, which are now such common- 
places to us, were then quite undreamed of. The 
moon displayed a patchy surface, and that was all ; 
her craters and ring-mountains were surprises as yet 
in store for men. Nothing of course was known 
about the surfaces of the planets. These objects had 
indeed no particular characteristics to distinguish them 
from the great host of the stars, except that they con- 
tinually changed their positions in the sky while the 
rest did not. The stars themselves were considered 
as fixed inalterably upon the vault of heaven. The 
sun, moon, and planets apparently moved about in the 
intermediate space, supported in their courses by 
strange and fanciful devices. The idea of satellites 
was as yet unknown. Comets were regarded as 

55 



Astronomy of To-day 

celestial portents, and meteors as small conflagrations 
taking place in the upper air. 

In the entire absence of any knowledge with regard 
to the actual sizes ^and distances of the various celes- 
tial bodies, men naturally considered them as small ; 
and, concluding that they \vere comparatively near, 
assigned to them in consequence a permanent con- 
nection with terrestial affairs. Thus arose the quaint 
and erroneous beliefs of astrology, according to which 
the events which took place upon our earth were con- 
sidered to depend upon the various positions in which 
the planets, for instance, found themselves from time 
to time. 

It must, however, be acknowledged that the study 
of astrology, fallacious though its conclusions were, 
indirectly performed a great service to astronomy by 
reason of the accurate observations and diligent study 
of the stars which it entailed. 

We will now inquire into the means by which the 
distances and sizes of the celestial orbs have been 
ascertained, and see how it was that the ancients were 
so entirely in the dark in this matter. 

There are two distinct methods of finding out the 
distance at which any object happens to be situated 
from us. 

One method is by actual measurement. 

The other is by moving oneself a little to the right 
or left, and observing whether the distant object 
appears in any degree altered in position by our own 
change of place. 

One of the best illustrations of this relative change of 
position which objects undergo as a result of our own 
change of place, is to observe the landscape from the 

56 



Celestial Measurement 

window of a moving railway carriage. As we are 
borne rapidly along we notice that the telegraph posts 
which are set close to the line appear to fly past us in 
the contrary direction ; the trees, houses, and other 
things beyond go by too, but not so fast; objects a 
good way off displace slowly ; while some spire, or 
tall landmark, in the far distance appears to remain 
unmoved during a comparatively long time. 

Actual change of position on our own part is found 
indeed to be invariably accompanied by an apparent 
displacement of the objects about us, such apparent 
displacement as a result of our own change of posi- 
tion being known as ^^ parallax." The dependence 
between the two is so mathematically exact, that if 
we know the amount of our own change of place, 
and if we observe the amount of the consequent 
displacement of any object, we are enabled to cal- 
culate its precise distance from us. Thus it comes 
to pass that distances can be measured without the 
necessity of moving over them; and the breadth of 
a river, for instance, or the distance from us of a 
ship at sea, can be found merely by such means. 

It is by the application of this principle to the 
wider field of the sky that we are able to ascertain 
the distance of celestial bodies. We have noted that 
it requires a goodly change of place on our own 
part to shift the position in which some object in 
the far distance is seen by us. To two persons 
separated by, say, a few hundred yards, a ship upon 
the horizon will appear pretty much in the same 
direction. They would require, in fact, to be much 
farther apart in order to displace it sufficiently for 
the purpose of estimating their distance from it. It 

57 



Astronomy of To-day 

is the same with regard to the moon. Two observers, 
standing upon our earth, will require to be some 
thousands of miles apart in order to see the position 
of our satellite sufficiently altered with regard to the 
starry background, to give the necessary data upon 
which to ground their calculations. 

The change of position thus offered by one side 
of the earth's surface at a time is, however, not suffi- 
cient to displace any but the nearest celestial bodies. 
When we have occasion to go farther afield we have 
to seek a greater change of place. This we can get 
as a consequence of the earth's movement around 
the sun. Observations, taken several days apart, will 
show the effect of the earth's change of place during 
the interval upon the positions of the other bodies 
of our system. But when we desire to sound the 
depths of space beyond, and to reach out to measure 
the distance of the nearest star, we find ourselves 
at once thrown upon the greatest change of place 
which we can possibly hope for ; and this we get 
during the long journey of many millions of miles 
which our earth performs around the sun during 
the course of each year. But even this last change 
of place, great as it seems in comparison with ter- 
restial measurements, is insufficient to show any- 
thing more than the tiniest displacements in a paltry 
forty-three out of the entire host of the stars. 

We can thus realise at what a disadvantage the 
ancients were. The measuring instruments at their 
command were utterly inadequate to detect such 
small displacements. It was reserved for the tele- 
scope to reveal them ; and even then it required 
the great telescopes of recent times to show the 

58 



Celestial Measurement 

slight changes in the position of the nearer stars, 
which were caused by the earth's being at one time 
at one end of its orbit, and some six months later 
at the other end — stations separated from each other 
by a gulf of about one hundred and eighty-six 
millions of miles. 

The actual distances of certain celestial bodies 
being thus ascertainable, it becomes a matter of no 
great difficulty to determine the actual sizes of the 
measurable ones. It is a matter of every-day ex- 
perience that the size which any object appears to 
have, depends exactly upon the distance it is from 
us. The farther off it is the smaller it looks ; the 
nearer it is the bigger. If, then, an object which 
lies at a known distance from us looks such and 
such a size, we can of course ascertain its real 
dimensions. Take the moon, for instance. As we 
have already shown, we are able to ascertain its 
distance. We observe also that it looks a certain 
size. It is therefore only a matter of calculation 
to find what its actual dimensions should be, in order 
that it may look that size at that distance away. 
Similarly we can ascertain the real dimensions of 
the sun. The planets, appearing to us as points 
of light, seem at first to offer a difficulty ; but, by 
means of the telescope, we can bring them, as it 
were, so much nearer to us, that their broad ex- 
panses may be seen. We fail, however, signally 
with regard to the stars ; for they are so very distant, 
and therefore such tiny points of light, that our 
mightiest telescopes cannot magnify them sufficiently 
to show any breadth of surface. 

Instead of saying that an object looks a certain 

59 



Astronomy of To-day 

breadth across, such as a yard or a foot, a statement 
which would really mean nothing, astronomers speak 
of it as measuring a certain angle. Such angles are 
estimated in what are called '' degrees of arc " ; each 
degree being divided into sixty minutes, and each 
minute again into sixty seconds. Popularly con- 
sidered the moon and sun look about the same size, 
or, as an astronomer would put it, they measure 
about the same angle. This is an angle, roughly, 
of thirty-two minutes of arc ; that is to say, slightly 
more than half a degree. The broad expanse of 
surface which a celestial body shows to us, whether 
to the naked eye, as in the case of the sun and moon, 
or in the telescope, as in the case of other members 
of our system, is technically known as its '' disc." 



60 



CHAPTER VII 

ECLIPSES AND KINDRED PHENOMENA 

Since some members of the solar system are nearer 
to us than* others, and all are again much nearer 
than any of the stars, it must often happen that one 
celestial body will pass between us and another, and 
thus intercept its light for a while. The moon, being 
the nearest object in the universe, will, of course, 
during its motion across the sky, temporarily blot 
out every one of the others which happen to lie in 
its path. When it passes in this manner across the 
face of the sun, it is said to eclipse it. When it thus 
hides a planet or star, it is said to occult it. The 
reason .why a separate term is used for what is merely 
a case ;of obscuring light in exactly the same way, 
will be plain when one considers that the disc of the 
sun is almost of the same apparent size as that of 
the moon, and so the complete hiding of the sun can 
last but a few minutes at the most ; whereas a planet 
or a star looks so very small in comparison, that it 
is always entirely swallowed up for so7ne time when it 
passes behind the body of our satellite. 

The sun, of course, occults planets and stars in 
exactly the same manner as the moon does, but we 
cannot see these occupations on account of the blaze 
of sunlight. 

By reason of the small size which the planets look 

6i 



Astronomy of To-day 

when viewed with the naked eye, we are not able to 
note them in the act of passing over stars and so 
blotting them out ; but such occurrences may be seen 
in the telescope, for the planetary bodies then display 
broad discs. 

There is yet another occurrence of the same class 
which is known as a transit. This takes place when 
an apparently small body passes across the face of 
an apparently large one, the phenomenon being in 
fact the exact reverse of an occultation. As there is 
no appreciable body nearer to us than the moon, we 
can never see anything in transit across her disc. 
But since the planets Venus and Mercury are both 
nearer to us than the sun, they will occasionally be 
seen to pass across his face, and thus we get the 
well-known phenomena called Transits of Venus and 
Transits of Mercury. 

As the satellites of Jupiter are continually revolving 
around him, they will often pass behind or across 
his disc. Such occultations and transits of satellites 
can be well observed in the telescope. 

There is, however, a way in which the light of a 
celestial body may be obscured without the necessity 
of its being hidden from us by one nearer. It will 
no doubt be granted that any opaque object casts 
a shadow when a strong light falls directly upon it. 
Thus the earth, under the powerful light which is 
directed upon it from the sun, casts an extensive 
shadow, though we are not aware of the existence 
of this shadow until it falls upon something. The 
shadow which the earth casts is indeed not noticeable 
to us until some celestial body passes into it. As the 
sun is very large, and the earth in comparison very 

62 



Eclipses and Kindred Phenomena 

small; the shadow thrown by the earth is compara- 
tively short, and reaches out in space for only about 
a million miles. There is no visible object except 
the moon, which circulates within that distance from 
our globe, and therefore she is the only body which 
can pass into this shadow. Whenever such a thing 
happens, her surface at once becomes dark, for the 
reason that she never emits any light of her own, 
but merely reflects that of the sun. As the moon is 
continually revolving around the earth, one would be 
inclined to imagine that once in every month, namely 
at what is C2L\led full moon, when she is on the other 
side of the earth with respect to the sun, she ought 
to pass through the shadow in question. But this 
does not occur every time, because the moon's orbit 
is not quite zipon the same plane with the earth's. It 
thus happens that time after time the moon passes 
clear of the earth's shadow, sometimes above it, and 
sometimes below it. It is indeed only at intervals of 
about six months that the moon can be thus obscured. 
This darkening of her light is known as an eclipse of 
the moon. It seems a great pity that custom should 
oblige us to employ the one term ^'eclipse" for this 
and also for the quite different occurrence, an eclipse 
of the sun ; in which the sun's face is hidden as a 
consequence of the moon's body coming directly 
between it and our eyes. 

The popular mind seems always to have found it 
more difficult to grasp the causes of an eclipse of 
the moon than an eclipse of the sun. As Mr. J. E. 
Gore^ puts it: '^The darkening of the sun's light 
by the interposition of the moon's body seems more 

^ Astronomical Essays {'^. 40), London, 1907. 

63 



Astronomy of To-day 

obvious than the passing of the moon through the 
earth's shadow." 

Eclipses of the moon furnish striking spectacles, 
but really add little to our knowledge. They exhibit, 
however, one of the most remarkable evidences of 
the globular shape of our earth ; for the outline of 
its shadow when seen creeping over the moon's 
surface is always circular. 

Eclipses of the Moon, or Lunar Eclipses, as they 
are also called, are of two kinds — Totals and Partial. 



Earlh 




Fig. 3. — Total and Partial Eclipses of the Moon. The Moon 
is here shown in two positions ; i.e. entirely plunged in 
the earth's shadow and therefore totally eclipsed, and only 
partly plunged in it or partially eclipsed. 

In a total lunar eclipse the moon passes entirely into 
the earth's shadow, and the whole of her surface is 
consequently darkened. This darkening lasts for 
about two hours. In a partial lunar eclipse, a portion 
only of the moon passes through the shadow^, and 
so only part of her surface is darkened (see Fig. 3). 
A very striking phenomenon during a total eclipse 
of the moon^ is that the darkening of the lunar 
surface is usually by no means so intense as one 
would expect, w^hen one considers that the sunlight 
at that time should be wholly cut off from it. The 
occasions indeed upon which the moon has com- 

64 



Eclipses and Kindred Phenomena 

pletely disappeared from view during the progress 
of a total lunar eclipse are very rare. On the majority 
of these occasions she has appeared of a coppery-red 
colour, while sometimes she has assumed an ashen 
hue. The explanations of these variations of colour 
is to be found in the then state of the atmosphere 
which surrounds our earth. When those portions 
of our earth's atmosphere through which the sun's 
rays have to filter on their way towards the moon 
are free from watery vapour, the lunar surface will 
be tinged with a reddish light, such as we ordinarily 
experience at sunset when our air is dry. The ashen 
colour is the result of our atmosphere being laden 
with watery vapour, and is similar to what we see 
at sunset when rain is about. Lastly, when the air 
around the earth is thickly charged with cloud, no 
light at all can pass ; and on such occasions the 
moon disappears altogether for the time being from 
the night sky. 

Eclipses of the Sufiy otherwise known as Solar 
Eclipses, are divided into Totals Partial, and Annular. 
A total eclipse of the sun takes place when the moon 
comes between the sun and the earth, in such a 
manner that it cuts off the sunlight entirely for the 
time being from 2, portion of the earth's surface. A 
person situated in the region in question will, there- 
fore, at that moment find the sun temporarily blotted 
out from his view by the body of the moon. Since 
the moon is a very much smaller body than the sun, 
and also very much the nearer to us of the two, it 
will readily be understood that the portion of the 
earth from which the sun is seen thus totally eclipsed 
will be of small extent. In places not very distant 

65 E 



Astronomy of To-day 

from this region, the moon will appear so much 
shifted in the sky that the sun will be seen onl}^ 
partially eclipsed. The moon being in constant 
movement round the earth, the portion of the earth's 
surface from which an eclipse is seen as total will 
be always a comparatively narrow band lying roughly 
from west to east. This band, known as the track 
of totality^ can, at the utmost, never be more than 
about 165 miles in width, and as a rule is very much 
less. For about 2000 miles on either side of it the 
sun is seen partially eclipsed. Outside these limits 
no eclipse of any kind is visible, as from such regions 
the moon is not seen to come in the way of the 
sun (see Fig. 4 (i.), p. 6^), 

It may occur to the reader that eclipses can also 
take place in the course of which the positions, where 
the eclipse would ordinarily be seen as total, will 
lie outside the surface of the earth. Such an eclipse 
is thus not dignified with the name of total eclipse, 
but is called 2, partial eclipse, because from the earth's 
surface the sun is only seen partly eclipsed at the 
utmost (see Fig. 4 (ii.), p. 67). 

An Annular eclipse is an eclipse which just fails 
to become total for yet another reason. We have 
pointed out that the orbits of the various members 
of the solar system are not circular, but oval. Such 
oval figures, it will be remembered, are technically 
known as ellipses. In an elliptic orbit the controlling 
body is situated not in the middle of the figure, but 
rather towards one of the ends ; the actual point 
which it occupies being known as the focus. The 
sun being at the focus of the earth's orbit, it follows 
that the earth is, at times, a little nearer to him than 

66 



Eclipses and Kindred Phenomena 

at others. The sun will therefore appear to us to 
vary a little in size, looking sometimes slightly larger 
than at other times. It is so, too, with the moon, at 
the focus of whose orbit the earth is situated. She 
therefore also appears to us at times to vary slightly 
in size. The result is that when the sun is eclipsed 





(i.) Total Eclipse of the Sun. 





Moon 



(ii, ) Partial Eclipse of the Sun. 

Fig. 4. — ^Total and Partial Eclipses of the Sun. From the 
position A the Sun cannot be seen, as it is entirely 
blotted out by the Moon. From B it is seen partially 
blotted out, because the Moon is to a certain degree in 
the way. From C no eclipse is seen, because the Moon 
does not come in the way. 

It is to be noted that in a Partial Eclipse of the Sun, 
the position A lies outside the surface of the Earth. 

by the moon, and the moon at the time appears the 
larger of the two, she is able to blot out the sun 
completely, and so we can get a total eclipse. But 
when, on the other hand, the sun appears the larger, 
the eclipse will not be quite total, for a portion of the 
sun's disc will be seen protruding all around the 
moon like a ring of light. This is what is known as 

67 



Astronomy of To-day 

an annular eclipse, from the Latin word annulusj 
which means a ring. The term is consecrated by 
long usage, but it seems an unfortunate one on 
account of its similarity to the word '^ annual." The 
Germans speak of this kind of eclipse as ^' ring- 
formed," which is certainly much more to the point. 

There can never be a year without an eclipse of the 
sun. Indeed there must be always two such eclipses 
at least during that period, though there need be no 
eclipse of the moon at all. On the other hand, the 
greatest number of eclipses which can ever take place 
during a year are seven ; that is to say, either 
five solar eclipses and two lunar, or four solar and 
three lunar. This general statement refers merely 
to eclipses in their broadest significance, and informs 
us in no way whether they will be total or partial. 

Of all the phenomena which arise from the hiding 
of any celestial body by one nearer coming in the 
way, a total eclipse of the sun is far the most im- 
portant. It is, indeed, interesting to consider how 
much poorer modern astronomy would be but for 
the extraordinary coincidence which makes a total 
solar eclipse just possible. The sun is about 400 
times farther off from us than the moon, and 
enormously greater than her in bulk. Yet the two 
are relatively so distanced from us as to look about 
the same size. The result of this is that the moon, 
as has been seen, can often blot out the sun entirely 
from our view for a short time. When this takes 
place the great blaze of sunlight which ordinarily 
dazzles our eyes is completely cut off, and we are 
thus enabled, unimpeded, to note what is going on in 
the immediate vicinity of the sun itself. 

68 



Eclipses and Kindred Phenomena 

In a total solar eclipse, the time which elapses 
from the moment when the moon's disc first begins 
to impinge upon that of the sun at his western edge 
until the eclipse becomes total, lasts about an hour. 
During all this time the black lunar disc may be 
watched making its way steadily across the solar 
face. Notwithstanding the gradual obscuration of 
the sun, one does not notice much diminution of light 
until about three-quarters of his disc are covered. 
Then a wan, unearthly appearance begins to pervade 
all things, the temperature falls noticeably, and 
nature seems to halt in expectation of the coming of 
something unusual. The decreasing portion of sun 
becomes more and more narrow, until at length it is 
reduced to a crescent-shaped strip of exceeding fine- 
ness. Strange, ill-defined, flickering shadows (known 
as " Shadow Bands ") may at this moment be seen 
chasing each other across any white expanse such 
as a wall, a building, or a sheet stretched upon the 
ground. The western side of the sky has now 
assumed an appearance dark and lowering, as if a 
rainstorm of great violence were approaching. This 
is caused by the mighty mass of the lunar shadow 
sweeping rapidly along. It flies onward at the terrific 
velocity, of about half a mile a second. 

If the gradually diminishing crescent of sun be 
now watched through a telescope, the observer will 
notice that it does not eventually vanish all at once, 
as he might have expected. Rather, it breaks up 
first of all along its length into a series of brilliant 
dots, known as '^ Baily's Beads." The reason of this 
phenomenon is perhaps not entirely agreed upon, 
but the majority of astronomers incline to the opinion 

69 



Astronomy of To-day 

that the so-called ''beads" are merely the last rem- 
nants of sunlight peeping between those lunar 
mountain peaks which happen at the moment to 
fringe the advancing edge of the moon. The beads 
are no sooner formed than they rapidly disappear 
one after the other, after which no portion of the 
solar surface is left to view, and the eclipse is now 
total (see Fig. 5). 





/n a total Eclipse /n an annu/arFclipse 

Fig. 5.—" Baily's Beads." 

But with the disappearance of the sun there springs 
into view a new and strange appearance, ordinarily 
unseen because of the blaze of sunlight. It is a kind 
of aureole, or halo, pearly white in colour, which is 
seen to surround the black disc of the moon. This 
white radiance is none other than the celebrated 
phenomenon widely known as the Solar Corona. It 
was once upon a time thought to belong to the 
moon, and to be perhaps a lunar atmosphere illumi- 
nated by the sunlight shining through it from behind. 
But the suddenness with which the moon always 
blots out stars when occulting them, has amply 

70 



Eclipses and Kindred Phenomena 

proved that she possesses no atmosphere worth 
speaking about. It is now, however, satisfactorily 
determined that the corona belongs to the sun, for 
during the short time that it remains in view the black 
body of the moon can be seen creeping across it. 

All the time that the total phase (as it is called) 
lasts, the corona glows with its pale unearthly light, 
shedding upon the earth's surface an illumination 
somewhat akin to full moonlight. Usually the planet 
Venus and a few stars shine out the while in the 
darkened heaven. Meantime around the observer 
animal and plant life behave as at nightfall. Birds 
go to roost, bats fly out, worms come to the surface 
of the ground, flowers close up. In the Norwegian 
eclipse of 1896 fish were seen rising to the surface 
of the water. When the total phase at length is over, 
and the moon in her progress across the sky has 
allowed the brilliant disc of the sun to spring into view 
once more at the other side, the corona disappears. 

There is another famous accompaniment of the 
sun which partly reveals itself during total solar 
eclipses. This is a layer of red flame which closely 
envelops the body of the sun and lies between it 
and the corona. This layer is known by the name 
of the Chromosphere. Just as at ordinary times we 
cannot see the corona on account of the blaze of 
sunlight, so are we likewise unable to see the chromo- 
sphere because of the dazzling white light which 
shines through from the body of the sun underneath 
and completely overpowers it. When, however, 
during a solar eclipse, the lunar disc has entirely 
hidden the brilliant face of the sun, we are still able 
for a few moments to see an edgewise portion of the 

71 



Astronomy of To-day 

chromosphere in the form of a narrow red strip, 
fringing the advancing border of the moon. Later 
on, just before the moon begins to uncover the face 
of the sun from the other side, we may again get a 
view of a strip of chromosphere. 

The outer surface of the chromosphere is not by 
any means even. It is rough and billowy, like the sur- 
face of a storm-tossed sea. Portions of it, indeed, rise 
at times to such heights that they may be seen stand- 
ing out like blood-red points around the black disc 
of the moon, and remain thus during a good part of 
the total phase. These projections are known as the 
Solar Prominences. In the same way as the corona, 
the chromosphere and prominences were for a time 
supposed to belong to the moon. This, however, 
was soon found not to be the case, for the lunar disc 
was noticed to creep slowly across them also. 

The total phase, or "totality," as it is also called, 
lasts for different lengths of time in different eclipses. 
It is usually of about two or three minutes' duration, 
and at the utmost it can never last longer than about 
eight minutes. 

When totality is over and the corona has faded 
away, the moon's disc creeps little by little from the 
face of the sun, light and heat returns once more to 
the earth, and nature recovers gradually from the 
gloom in which she has been plunged. About an hour 
after totality, the last remnant of moon draws away 
from the solar disc, and the eclipse is entirely at 
an end. 

The corona, the chromosphere, and the promi- 
nences are the most important of these accompani- 
ments of the sun which a total eclipse reveals to us. 

72 



Eclipses and Kindred Phenomena 

Our further consideration of them must, however, be 
reserved for a subsequent chapter, in which the sun 
will be treated of at length. 

Every one who has had the good fortune to see 
a total eclipse of the sun will, the writer feels sure, 
agree with the verdict of Sir Norman Lockyer that 
it is at once one of the "grandest and most awe- 
inspiring sights " which man can witness. Needless 
to say, such an occurrence used to cause great con- 
sternation in less civilised ages ; and that it has not 
in modern times quite parted with its terrors for 
some persons, is shown by the fact that in Iowa, in 
the United States, a woman died from fright during 
the eclipse of 1869. 

To the serious observer of a total solar eclipse 
every instant is extremely precious. Many distinct 
observations have to be crowded into a time all too 
limited, and this in an eclipse-party necessitates 
constant rehearsals in order that not a moment may 
be wasted when the longed-for totality arrives. Such 
preparation is very necessary ; for the rarity and 
uncommon nature of a total eclipse of the sun, 
coupled with its exceeding short duration, tends to 
flurry the mind, and to render it slow to seize upon 
salient points of detail. And, even after every pre- 
caution has been taken, weather possibilities remain 
to be reckoned with, so that success is rather a 
lottery. 

Above all things, therefore, a total solar eclipse is 
an occurrence for the proper utilisation of which 
personal experience is of absolute necessity. It was 
manifestly out of the question that such experience 
could be gained by any individual in early times, 

73 



Astronomy of To-day 

as the imperfection of astronomical theory and 
geographical knowledge rendered the predicting of 
the exact position of the track of totality well-nigh 
impossible. Thus chance alone would have enabled 
one in those days to witness a total phase, and the 
probabilities, of course, were much against a second 
such experience in the span of a life-time. And even 
in more modern times, when the celestial motions 
had come to be better understood, the difficulties of 
foreign travel still were in the way ; for it is, indeed, 
a notable fact that during many years following the 
invention of the telescope the tracks were placed for 
the most part in far-off regions of the earth, and 
Europe was visited by singularly few total solar 
eclipses. Thus it came to pass that the building up 
of a body of organised knowledge upon this subject 
was greatly delayed. 

Nothing perhaps better shows the soundness of 
modern astronomical theory than the almost exact 
agreement of the time predicted for an eclipse with 
its actual occurrence. Similarly, by calculating back- 
wards, astronomers have discovered the times and 
seasons at which many ancient eclipses took place, 
and valuable opportunities have thus arisen for 
checking certain disputed dates in history. 

It should not be omitted here that the ancients 
were actually able, in a rough way, to predict eclipses. 
The Chaldean astronomers had indeed noticed very 
early a curious circumstance, i.e, that eclipses tend 
to repeat themselves after a lapse of slightly more 
than eighteen years. 

In this connection it must, however, be pointed 
out, in the first instance, that the eclipses which 

74 



Eclipses and Kindred Phenomena 

occur in any particular year are in no way associated 
with those which occurred in the previous year. 
In other words, the mere fact that an eclipse takes 
place upon a certain day this year will not bring 
about a repetition of it at the same time next year. 
However, the nicely balanced behaviour of the solar 
system, an equilibrium resulting from aeons of orbital 
ebb and flow, naturally tends to make the members 
which compose that family repeat their ancient com- 
binations again and again ; so that after definite 
lapses of time the same order of things will almost 
exactly recur. Thus, as a consequence of their 
beautifully poised motions, the sun, the moon, and 
the earth tend, after a period of i8 years and io\ 
days,^ to occupy very nearly the same positions with 
regard to each other. The result of this is that, 
during each recurring period, the eclipses comprised 
within it will be repeated in their order. 

To give examples : — 

The total solar eclipse of August 30, 1905, was a 
repetition of that of August 19, 1887. 

The partial solar eclipse of February 23, 1906, 
corresponded to that which took place on February 
II, 1888. 

The annular eclipse of July 10, 1907, was a recur- 
rence of that of June 28, 1889. 

In this way we can go on until the eighteen year 
cycle has run out, and we come upon a total solar 

1 In some cases the periods between the dates of the corresponding 
eclipses appem- to include a greater number of days than ten ; but this is 
easily explained when allowance is made for intervening leap years (in 
each of which an extra day has of course been added), and also for 
variations in local time. 

75 



Astronomy of To-day 

eclipse predicted for September lo, 1923, which will 
repeat the above-mentioned ones of 1905 and 1887 ; 
and so on too with the others. 

From mere observation alone, extending no doubt 
over many ages, those time-honoured watchers of 
the sky, the early Chaldeans, had arrived at this 
remarkable generalisation ; and they used it for the 
rough prediction of eclipses. To the period of re- 
currence they give the name of ^^ Saros." 

And here we find ourselves led into one of the 
most interesting and fascinating by-paths in as- 
tronomy, to which writers, as a rule, pay all too 
little heed. 

In order not to complicate matters unduly, the 
recurrence of solar eclipses alone will first be dealt 
with. This limitation will, however, not affect the 
arguments in the slightest, and it will be all the 
more easy in consequence to show their application 
to the case of eclipses of the moon. 

The reader will perhaps have noticed that, with 
regard to the repetition of an eclipse, it has been 
stated that the conditions which bring it on at each 
recurrence are reproduced almost exactly. Here, then, 
lies the crux of the situation. For it is quite evident 
that were the conditions exactly reproduced, the re- 
currences of each eclipse would go on for an in- 
definite period. For instance, if the lapse of a saros 
period found the sun, moon, and earth again in the 
precise relative situations which they had previously 
occupied, the recurrences of a solar eclipse would 
tend to duplicate its forerunner with regard to the 
position of the shadow upon the terrestrial surface. 
But the conditions not being exactly reproduced, the 

76 



Eclipses and Kindred Phenomena 

shadow-track does not pass across the earth in 
quite the same regions. It is shifted a little, so to 
speak ; and each time the eclipse comes round it is 
found to be shifted a little farther. Every solar 
eclipse has therefore a definite ^Mife" of its own upon 
the earth, lasting about 1150 years, or 64 saros 
returns, and working its way little by little across our 
globe from north to south, or from south to north, 
as the case may be. Let us take an imaginary 
example. A partial eclipse occurs, say, somewhere 
near the North Pole, the edge of the ^' partial " 
shadow just grazing the earth, and the ^' track of 
totality" being as yet cast into space. Here we have 
the beginning of a series. At each saros recurrence 
the partial shadow encroaches upon a greater extent 
of earth-surface. At length, in its turn, the track of 
totality begins to impinge upon the earth. This 
track streaks across our globe at each return of the 
eclipse, repeating itself every time in a slightly more 
southerly latitude. South and south it moves, pass- 
ing in turn the Tropic of Cancer, the Equator, the 
Tropic of Capricorn, until it reaches the South 
Pole ; after which it touches the earth no longer, 
but is cast into space. The rear portion of the 
partial shadow, in its turn, grows less and less in 
extent ; and it too in time finally passes off. Our 
imaginary eclipse series is now no more — its '' life " 
has ended. 

We have taken, as an example, an eclipse series 
moving from north to south. We might have taken 
one moving from south to north, for they progress 
in either direction. 

From the description just given the reader might 

77 



Astronomy of To-day 

suppose that, if the tracks of totality of an eclipse 
series were plotted upon a chart of the world, they 
would lie one beneath another like a set of steps. 
This is, however, not the case, and the reason is 
easily found. It depends upon the fact that the 
saros does not comprise an exact number of days, 
but includes, as we have seen, one-third of a day in 
addition. 

It will be granted, of course, that if the number of 
days was exact, the same parts of the earth would 
always be brought round by the axial rotation to 
front the sun at the moment of the recurrence of the 
eclipse. But as there is still one-third of a day to 
complete the saros period, the earth has yet to make 
one-third of a rotation upon its axis before the eclipse 
takes place. Thus at every recurrence the track of 
totality finds itself placed one-third of the earth's 
circumference to the westward. Three of the recur- 
rences will, of course, complete the circuit of the 
globe ; and so the fourth recurrence will duplicate 
the one which preceded it, three saros returns, or 
54 years and i month before. This duplication, as 
we have already seen, will, however, be situated in 
a latitude to the south or north of its predecessor, 
according as the eclipse series is progressing in a 
southerly or northerly direction. 

Lastly, every eclipse series, after working its way 
across the earth, will return^ again to go through the 
same process after some 12,000 years ; so that, at 
the end of that great lapse of time, the entire ^' life " 
of every eclipse should repeat itself, provided that the 
conditions of the solar system have not altered ap- 
preciably during the interval. 

78 



Eclipses and Kindred Phenomena 

We are now in a position to consider this gradual 
southerly or northerly progress of eclipse recurrences 
in its application to the case of eclipses of the moon. 
It should be evident that, just as in solar eclipses the 
lunar shadow is lowered or raised (as the case may 
be) each time it strikes the terrestrial surface, so in 
lunar eclipses will the body of the moon shift its 
place at each recurrence relatively to the position of 
the earth's shadow. Every lunar eclipse, therefore, 
will commence on our satellite's disc as a partial 
eclipse at the northern or southern extremity, as the 
case may be. Let us take, as an example, an imagi- 
nary series of eclipses of the moon progressing from 
north to south. At each recurrence the partial 
phase will grow greater, its boundary encroaching 
more and more to the southward, until eventually 
the whole disc is enveloped by the shadow, and the 
eclipse becomes total. It will then repeat itself as 
total during a number of recurrences, until the entire 
breadth of the shadow has been passed through, and 
the northern edge of the moon at length springs out 
into sunlight. This illuminated portion will grow 
more and more extensive at each succeeding return, 
the edge of the shadow appearing to recede from 
it until it finally passes off at the south. Similarly, 
when a lunar eclipse commences as partial at the 
south of the moon, the edge of the shadow at each 
subsequent recurrence finds itself more and more to 
the northward. In due course the total phase will 
supervene, and will persist during a number of re- 
currences until the southerly trend of the moon 
results in the uncovering of the lunar surface at the 
south. Thus, as the boundary of the shadow is left 

79 



Astronomy of To-day 

more and more to the northward, the illuminated 
portion on the southern side of the moon becomes at 
each recurrence greater and the darkened portion on 
the northern side less, until the shadow eventually 
passes off at the north. 

The ^Mife" of an eclipse of the moon happens, for 
certain reasons, to be much shorter than that of an 
eclipse of the sun. It lasts during only about 860 
years, or 48 saros returns. 

Fig. 6, p. 81, is a map of the world on Mercator's 
Projection, showing a portion of the march of the 
total solar eclipse of August 30, 1905, across the sur- 
face of the earth. The projection in question has 
been employed because it is the one with which people 
are most familiar. This eclipse began by striking the 
neighbourhood of the North Pole in the guise of a 
partial eclipse during the latter part of the reign of 
Queen Elizabeth, and became total on the earth for 
the first 'time on the 24th of June 1797. Its next 
appearance was on the 6th of July 181 5. It has not 
been possible to show the tracks of totality of these 
two early visitations on account of the distortion of 
the polar regions consequent on the fiction of Mer- 
cator's Projection. It is therefore made to commence 
with the track of its third appearance, viz. on July 
17, 1833. In consequence of those variations in the 
apparent sizes of the sun and moon, which result, as 
we have seen, from the variations in their distances 
from the earth, this eclipse will change from a total 
into an annular eclipse towards the end of the twenty- 
first century. By that time the track will have passed 
to the southern side of the equator. The track 

80 



Eclipses and Kindred Phenomena 




8i 



F 



Astronomy of To-day 

will eventually leave the earth near the South Pole 
about the beginning of the twenty-sixth century, and 
the rear portion of the partial shadow will in its 
turn be clear of the terrestrial surface by about 2700 
A.D., when the series comes to an end. 



82 



CHAPTER VIII 

FAMOUS ECLIPSES OF THE SUN 

What is thought to be the earhest reference to an 
edipse comes down to us from the ancient Chinese 
records^ and is over four thousand years old. The 
ecHpse in question was a solar one, and occurred, 
so far as can be ascertained, during the twenty-second 
century B.C. The story runs that the two state 
astronomers, Ho and Hi by name, being exceedingly 
intoxicated, were unable to perform their required 
duties, which consisted in superintending the custo- 
mary rites of beating drums, shooting arrows, and 
the like, in order to frighten away the mighty dragon 
which it was believed was about to swallow up the 
Lord of Day. This eclipse seems to have been only 
partial ; nevertheless a great turmoil ensued, and the 
two astronomers were put to death, no doubt with 
the usual celestial cruelty. 

The next eclipse mentioned in the Chinese annals 
is also a solar eclipse, and appears to have taken 
place more than a thousand years later, namely in 
776 B.C. Records of similar eclipses follow from the 
same source ; but as they are mere notes of the 
events, and do not enter into any detail, they are of 
little interest. Curiously enough the Chinese have 
taken practically no notice of eclipses of the moon, 
but have left us a comparatively careful record of 

83 



Astronomy of To-day 

comets, which has been of value to modern astro- 
nomy. 

The earliest mention of a total eclipse of the sun 
(for it should be noted that the ancient Chinese 
eclipse above-mentioned was merely partial) was de- 
ciphered in 1905, on a very ancient Babylonian tablet, 
by Mr. L. W. King of the British Museum. This 
eclipse took place in the year 1063 B.C. 

Assyrian tablets record three solar eclipses which 
occurred between three and four hundred years later 
than this. The first of these was in 763 B.C. ; the 
total phase being visible near Nineveh. 

The next record of an eclipse of the sun comes 
to us from a Grecian source. This eclipse took place 
in 585 B.C.; and has been the subject of much in- 
vestigation. Herodotus, to whom we are indebted 
for the account, tells us that it occurred during a 
battle in a war which had been waging for some 
years between the Lydians and Medes. The sudden 
coming on of darkness led to a termination of the 
contest, and peace was afterwards made between the 
combatants. The historian goes on to state that the 
eclipse had been foretold by Thales, who is looked 
upon as the Founder of Grecian astronomy. This 
eclipse is in consequence known as the ^'Eclipse of 
Thales." It would seem as if that philosopher were 
acquainted with the Chaldean saros. 

The next solar eclipse worthy of note was an 
annular one, and occurred in 431 B.C., the first year 
of the Peloponnesian War. Plutarch relates that the 
pilot of the ship, which was about to convey Pericles 
to the Peloponnesus, was very much frightened by 
it ; but Pericles calmed him by holding up a cloak 

84 



Famous Eclipses of the Sun 

before his eyes, and saying that the only difference 
between this and the ecHpse was that something 
larger than the cloak prevented his seeing the sun 
for the time being. 

An eclipse of great historical interest is that known 
as the ^' Eclipse of Agathocles/' which occurred on 
the morning of the 15th of August, 310. B.C. Aga- 
thocles, Tyrant of Syracuse, had been blockaded in 
the harbour of that town by the Carthaginian fleet, 
but effected the escape of his squadron under cover 
of night, and sailed for Africa in order to invade the 
enemy's territory. During the following day he and 
his vessels experienced a total eclipse, in which 'May 
wholly put on the appearance of night, and the stars 
were seen in all parts of the sky." 

A few solar eclipses are supposed to be referred to 
in early Roman history, but their identity is very 
doubtful in comparison with those which the Greeks 
have recorded. Additional doubt is cast upon them by 
the fact that they are usually associated with famous 
events. The birth and death of Romulus, and the 
Passage of the Rubicon by Julius Caesar, are stated 
indeed to have been accompanied by these marks of 
the approval or disapproval of the gods ! 

Reference to our subject in the Bible is scanty. 
Amos viii. 9 is thought to refer to the Nineveh eclipse 
of 763 B.C., to which allusion has already been made ; 
while the famous episode of Hezekiah and the shadow 
on the dial of Ahaz has been connected with an 
eclipse which was partial at Jerusalem in 689 B.C. 

The first solar eclipse, recorded during the Christian 
Era, is known as the '^ Eclipse of Phlegon,"from the fact 
that we are indebted for the account to a pagan writer 

8s 



Astronomy of To-day 

of that name. This eclipse took place in a.d. 29, and 
the total phase was visible a little to the north of 
Palestine. It has sometimes been confounded with 
the ^' darkness of the Crucifixion/' which event took 
place near the date in question ; but it is sufficient 
here to say that the Crucifixion is well known to have 
occurred during the Passover of the Jews, which is 
always celebrated at ih.Qfull moon, whereas an eclipse 
of the sun can only take place at new moon. 

Dion Cassius, commenting on the Emperor Claudius 
about the year A.D. 45, writes as follows : — 

^'As there was going to be an eclipse on his birth- 
day, through fear of a disturbance, as there had been 
other prodigies, he put forth a public notice, not only 
that the obscuration would take place, and about the 
time and magnitude of it, but also about the causes 
that produce such an event." 

This is a remarkable piece of information ; for the 
Romans, an essentially military nation, appear hitherto 
to have troubled themselves very little about astro- 
nomical matters, and were content, as we have seen, 
to look upon phenomena, like eclipses, as mere 
celestial prodigies. 

What is thought to be the first definite mention of 
the solar corona occurs in a passage of Plutarch. 
The eclipse to which he refers is probably one which 
took place in A.D. 71. He says that the obscuration 
caused by the moon ^^ has no time to last and no ex- 
tensiveness, but some light shows itself round the 
sun's circumference, which does not allow the dark- 
ness to become deep and complete." No further 
reference to this phenomenon occurs until near the end 
of the sixteenth century. It should, however, be here 

86 



Famous Eclipses of the Sun 

mentioned that Mr. E. W. Maunder has pointed out 
the probability 1 that we have a very ^ancient symboUc 
representation of the corona in the ^^ winged circle," 
" winged disc/' or '^ ring with wings/' as it is variously 
called, which appears so often upon Assyrian and 




Fig. 7. — The " Ring with Wings." The upper is the Assyrian 
form of the symbol, the lower the Egyptian. (From 
Knoidedge.) Compare the form of the corona on Plate VII. 
(B), p. 142. 

Egyptian monuments, as the symbol of the Deity 

(Fig- 7)- 

The first solar eclipse recorded to have been seen in 

England is that of a.d. 538, mention of which is found 

in the Anglo-Saxon Chronicle. The track of totality 

did not, however, come near our islands, for only 

two-thirds of the sun's disc were eclipsed at London. 

1 Knoivledge^ vol. xx. p. 9, January 1897. 

87 



Astronomy of To-day 

In 840 a great eclipse took place in Europe, which 
was total for more than five minutes across what is 
now Bavaria. Terror at this eclipse is said to have 
hastened the death of Louis le Debonnaire, Emperor 
of the West, who lay ill at Worms. 

In 878 — temp. King Alfred — an eclipse of the sun 
took place which was total at London. From this 
until 171 5 no other eclipse was total at London 
itself ; though this does not apply to other portions 
of England. 

An eclipse, generally known as the ^' Eclipse of 
Stiklastad," is said to have taken place in 1030, during 
the sea-fight in which Olaf of Norway is supposed to 
have been slain. Longfellow, in his Saga of King 

Olaf, has it that 

"The Sun hung red 
As a drop of blood," 

but, as in the case of most poets, the dramatic value of 
an eclipse seems to have escaped his notice. 

In the year 1140 there occurred a total eclipse of 
the sun, the last to be visible in England for more 
than five centuries. Indeed there have been only two 
such since — namely, those of 171 5 and 1724, to which 
we shall allude in due course. The eclipse of 1140 
took place on the 20th March, and is thus referred to 
in the Anglo-Saxon Chronicle : — 

^' In the Lent, the sun and the day darkened, about 
the noon-tide of the day, when men were eating, and 
they lighted candles to eat by. That was the 13th 
day before the calends of April. Men were very 
much struck with wonder." 

Several of the older historians speak of a ^'fearful 
eclipse " as having taken place on the morning of the 



Famous Eclipses of the Sun 

Battle of Crecy, 1346. Lingard, for instance, in his 
History of England, has as follows : — 

" Never, perhaps, were preparations for battle made 
under circumstances so truly awful. On that very 
day the sun suffered a partial eclipse : birds, in 
clouds, the precursors of a storm, flew screaming 
over the two armies, and the rain fell in torrents, 
accompanied by incessant thunder and lightning. 
About five in the afternoon the weather cleared up ; 
the sun in full splendour darted his rays in the eyes 
of the enemy." 

Calculations, however, show that no eclipse of the 
sun took place in Europe during that year. This 
error is found to have arisen from the mistranslation 
of an obsolete French word esclistre (lightning), which 
is employed by Froissart in his description of the battle. 

In 1598 an eclipse was total over Scotland and part 
of North Germany. It was observed at Torgau by 
Jessenius, an Hungarian physician, who noticed a 
bright light around the moon during the time of 
totality. This is said to be the first reference to the 
corona since that of Plutarch, to which we have 
already drawn attention. 

Mention of Scotland recalls the fact that an unusual 
number of eclipses happen to have been visible in 
that country, and the occult bent natural to the 
Scottish character has traditionalised a few of them 
in such terms as the "Black Hour" (an eclipse of 
1433), "Black Saturday" (the eclipse of 1598 which 
has been alluded to above), and "Mirk Monday" 
(1652). The track of the last-named also passed 
over Carrickfergus in Ireland, where it was observed 
by a certain Dr. Wybord, in whose account the 

89 



Astronomy of To-day 

term ^'corona" is first employed. This eclipse is 
the last which has been total in Scotland, and it is 
calculated that there will not be another eclipse seen 
as total there until the twenty-second century. 

An eclipse of the sun which took place on May 
30, 1612, is recorded as having been seen "through 
a tube." This probably refers to the then recent 
invention — the telescope. 

The eclipses which we have been describing are 
chiefly interesting from an historical point of view. 
The old mystery and confusion to the beholders seem 
to have lingered even into comparatively enlightened 
times, for we see how late it is before the corona 
attracts definite attention for the sake of itself alone. 

It is not a far cry from notice of the corona to that 
of other accompaniments of a solar eclipse. Thus 
the eclipse of 1706, the total phase of which was 
visible in Switzerland, is of great interest ; for it was 
on this occasion that the famous red prominences 
seem first to have been noted. A certain Captain 
Stannyan observed this eclipse from Berne in Switzer- 
land, and described it in a letter to Flamsteed, the 
then Astronomer Royal. He says the sun's " getting 
out of his eclipse was preceded by a blood-red streak 
of light from its left limb, which continued not longer 
than six or seven seconds of time ; then part of the 
Sun's disc appeared all of a sudden, as bright as 
Venus was ever seen in the night, nay brighter ; and 
in that very instant gave a Light and Shadow to 
things as strong as Moonlight uses to do." How 
little was then expected of the sun is, however, shown 
by Flamsteed's words, when communicating this in- 
formation to the Royal Society : — 

90 



Famous Eclipses of the Sun 

''The Captain is the first man I ever heard of 
that took notice of a Red Streak of Light preceding 
the Emersion of the Sun's body from a total EcHpse. 
And I take notice of it to you because it infers that 
the Moon has mi atmosphere ; and its short continu- 
ance of only six or seven seconds of time, tells us 
that its height is not more than the five or six hundredth 
part of her diameter ^ 

What a change has since come over the ideas of 
men ! The sun has proved a veritable mine of dis- 
covery, while the moon has 37^ielded up nothing new. 

The eclipse of 1715, the first total at London since 
that of 878, was observed by the famous astronomer, 
Edmund Halley, from the rooms of the Royal 
Society, then in Crane Court, Fleet Street. On 
this occasion both the corona and a red projection 
were noted. Halley further makes allusion to that 
curious phenomenon, which later on became cele- 
brated under the name of '' Baily's beads." It was 
also on the occasion of this eclipse that the earliest 
recorded drawings of the corona were made. Cam- 
bridge happened to be within the track of totality; 
and a certain Professor Cotes of that University, 
who is responsible for one of the drawings in ques- 
tion, forwarded them to Sir Isaac Newton together 
with a letter describing his observations. 

In 1724 there occurred an eclipse, the total phase 
of which was visible from the south-west of England, 
but not from London. The weather was unfavour- 
able, and the eclipse consequently appears to have 
been seen by only one person, a certain Dr. Stukeley, 
who observed it from Haraden Hill near Salisbur}^ 
Plain. This is the last eclipse of which the total 

91 



Astronomy of To-day 

phase was seen in any part of England. The next 
will not be until June 29, 1927, and will be visible 
along a line across North Wales and Lancashire. 
The discs of the sun and moon will just then be 
almost of the same apparent size, and so totality 
will be of extremely short duration ; in fact only 
a few seconds. London itself will not see a totality 
until the year 2151 — a circumstance which need 
hardly distress any of us personally ! 

It is only from the early part of the nineteenth 
century that serious scientific attention to eclipses 
of the sun can be dated. An annular eclipse, visible 
in 1836 in the south of Scotland, drew the careful 
notice of Francis Baily of Jedburgh in Roxburgh- 
shire to that curious phenomenon which we have 
already described, and which has ever since been 
known by the name of ^' Daily's beads." Spurred by 
his observation, the leading astronomers of the day 
determined to pay particular attention to a total 
eclipse, which in the year 1842 was to be visible in 
the south of France and the north of Italy. The 
public interest aroused on this occasion was also 
very great, for the region across which the track of 
totality was to pass was very populous, and inhabited 
by races of a high degree of culture. 

This eclipse occurred on the morning of the 8th 
July, and from it may be dated that great enthu- 
siasm with which total eclipses of the sun have ever 
since been received. Airy, our then Astronomer 
Royal, observed it from Turin ; Arago, the celebrated 
director of the Paris Observatory, from Perpignan 
in the south of France ; Francis Baily from Pavia ; 
and Sir John Herschel from Milan. The corona 

92 



Famous Eclipses of the Sun 

and three large red prominences were not only well 
observed by the astronomers, but drew tremendous 
applause from the watching multitudes. 

The success of the observations made during this 
eclipse prompted astronomers to pay similar attention 
to that of July 28, 1851, the total phase of which 
was to be visible in the south of Norway and Sweden, 
and across the east of Prussia. This eclipse was 
also a success, and it was now ascertained that the 
red prominences belonged to the sun and not to 
the moon ; for the lunar disc, as it moved onward, 
was seen to cover and to uncover them in turn. 
It was also noted that these prominences were merely 
uprushes from a layer of glowing gaseous matter, 
which was seen closely to envelop the sun. 

The total eclipse of July 18, i860, was observed 
in Spain, and photography was for the first time 
systematically employed in its observation.^ In the 
photographs taken the stationary appearance of both 
the corona and prominences with respect to the 
moving moon, definitely confirmed the view already 
put forward that they were actual appendages of 
the sun. 

The eclipse of August 18, 1868, the total phase of 
which lasted nearly six minutes, was visible in India, 
and drew thither a large concourse of astronomers. 
In this eclipse the spectroscope came to the front, 
and showed that both the prominences, and the 
chromospheric layer from which they rise, are com- 
posed of glowing vapours — chief among which is the 

^ The first photographic representation of the coro/ia had, however, been 
made during the eclipse of 1851. This was a daguerreotype taken by 
Dr. Eusch at Konigsberg in Prussia, 

93 



Astronomy of To-day 

vapour of hydrogen. The dhxct result of the obser- 
vations made on this occasion was the spectroscopic 
method of examining prominences at any time in full 
daylight, and without a total eclipse. This method, 
which has given such an immense impetus to the 
study of the sun, was the outcome of independent 
and simultaneous investigation on the part of the 
French astronomer, the late M. Janssen, and the Eng- 
lish astronomer. Professor (now Sir Norman) Lockyer, 
a circumstance strangely reminiscent of the discovery 
of Neptune. The principles on which the method 
was founded seem, however, to have occurred to Dr. 
(now Sir William) Huggins some time previously. 

The eclipse of December 22, 1870, was total for a 
little more than two minutes, and its track passed 
across the Mediterranean. M. Janssen, of whom 
mention has just been made, escaped in a balloon 
from then besieged Paris, taking his instruments with 
him, and made his way to Gran, in Algeria, in order 
to observe it ; but his expectations were disappointed 
by cloudy weather. The expedition sent out from 
England had the misfortune to be shipwrecked oiT 
the coast of Sicily. But the occasion was redeemed 
by a memorable observation made by the American 
astronomer, the late Professor Young, which revealed 
the existence of what is now known as the ^' Reversing 
Layer." This is a shallow layer of gases which lies 
immediately beneath the chromosphere. An illustra- 
tion of the corona, as it was seen during the above 
eclipse, will be found on Plate VII. (A), p. 142. 

In the eclipse of December 12, 1871, total across 
Southern India, the photographs of the corona ob- 
tained by Mr. Davis, assistant to Lord Lindsay (now 

94 



Famous Eclipses of the Sun 

the Earl of Crawford), displayed a wealth of detail 
hitherto unapproached. 

The eclipse of July 29, 1878, total across the western 
states of North America, was a remarkable success, 
and a magnificent view of the corona was obtained by 
the well-known American astronomer and physicist, 
the late Professor Langley, from the summit of Pike's 
Peak, Colorado, over 14,000 feet above the level of the 
sea. The coronal streamers were seen to extend to a 
much greater distance at this altitude than at points 
less elevated, and the corona itself remained visible 
during more than four minutes after the end of totality. 
It was, however, not entirely a question of altitude ; 
the coronal streamers were actually very much longer 
on this occasion than in most of the eclipses which 
had previously been observed. 

The eclipse of May 17, 1882, observed in Upper 
Egypt, is notable- from the fact that, in one of the 
photographs taken by Dr. Schuster at Sohag, a bright 
comet appeared near the outer limit of the corona 
(see Plate I., p. 96). The comet in question had not 
been seen before the eclipse, and was never seen after- 
wards. This is the third occasion on which attention 
has been drawn to a comet merely by a total eclipse. 
The first is mentioned by Seneca ; and the second by 
Philostorgius, in an account of an eclipse observed at 
Constantinople in A.D. 418. A fourth case of the kind 
occurred in 1893, when faint evidences of one of these 
filmy objects were found on photographs of the corona 
taken by the American astronomer, Professor Schae- 
berle, during the total eclipse of April 16 of that 
year. 

The eclipse of May 6, 1883, had a totality of over 

95 



Astronomy of To-day 

five minutes, but the central track unfortunately- 
passed across the Pacific Ocean, and the sole point of 
land available for observing it from was one of the 
Marquesas Group, Caroline Island, a coral atoll seven 
and a half miles long by one and a half broad. Never- 
theless astronomers did not hesitate to take up their 
posts upon that little spot, and were rewarded with 
good weather. 

The next eclipse of importance was that of April 
1 6, 1893. It stretched from Chili across South 
America and the Atlantic Ocean to the West Coast 
of Africa, and, as the weather was fine, many good 
results were obtained. Photographs were taken at 
both ends of the track, and these showed that the 
appearance of the corona remained unchanged during 
the interval of time occupied by the passage of the 
shadow across the earth. It was on the occasion of 
this eclipse that Professor Schaeberle found upon his 
photographs those traces of the presence of a comet, 
to which allusion has already been made. 

Extensive preparations were made to observe the 
eclipse of August 9, 1896. Totality lasted from two to 
three minutes, and the track stretched from Norway 
to Japan. Bad weather disappointed the observers, 
with the exception of those taken to Nova Zembla 
by Sir George Baden Powell in his yacht Otaria. 

The eclipse of January 22, 1898, across India vid 
Bombay and Benares, was favoured with good 
weather, and is notable for a photograph obtained 
by Mrs. E. W. Maunder, which showed a ray of the 
corona extending to a most unusual distance. 

Of very great influence in the growth of our know- 
ledge with regard to the sun, is the remarkable piece 

q6 




Plate I. The Total Eclipse of the Sun of May i7th, 1SS2 

A comet is here shown in the immediate neighbourhood of the corona. 
Drawn by Mr. W. H. Wesley from the photographs. 

(Page 05) 



Famous Eclipses of the Sun 

of good fortune by which the countries around the 
Mediterranean, so easy of access, have been favoured 
with a comparatively large number of total eclipses 
during the past sixty years. Tracks of totality have, 
for instance, traversed the Spanish peninsula on no 
less than five occasions during that period. Two of 
these are among the most notable eclipses of recent 
years, namely, those of May 28, 1900, and of August 
30, 1905. In the former the track of totality stretched 
from the western seaboard of Mexico, through the 
Southern States of America, and across the Atlantic 
Ocean, after which it passed over Portugal and 
Spain into North Africa. The total phase lasted for 
about a minute and a half, and the eclipse was well 
observed from a great many points along the line. A 
representation of the corona, as it appeared on this 
occasion, will be found on Plate VII. (B), p. 142. 

The track of the other eclipse to which we have 
alluded, i.e. that of August 30, 1905, crossed Spain 
about 200 miles to the northward of that of 1900. 
It stretched from Winnipeg in Canada, through 
Labrador, *and over the Atlantic ; then traversing 
Spain, it passed across the Balearic Islands, North 
Africa, and Egypt, and ended in Arabia (see Fig. 6, p. 
81). Much was to be expected from a comparison 
between the photographs taken in Labrador and 
Egypt on the question as to whether the corona 
would show any alteration in shape during the time 
that the shadow was traversing the intervening space 
— some 6000 miles. The duration of the total phase 
in this eclipse was nearly four minutes. Bad weather, 
however, interfered a good deal with the observations. 
It was not possible, for instance, to do anything at all 

97 ^ 



Astronomy of To-day 

in Labrador. In Spain the weather conditions were 
by no means favourable ; though at Burgos, where 
an immense number of people had assembled, the 
total phase was, fortunately, well seen. On the 
whole, the best results were obtained at Guelma in 
Algeria. The corona on the occasion of this eclipse 
was a very fine one, and some magnificent groups of 
prominences were plainly visible to the naked eye 
(see the Frontispiece). 

The next total eclipse after that of 1905 was one 
which occurred on January 14, 1907. It passed 
across Central Asia and Siberia, and had a totality 
lasting two and a half minutes at most ; but it was 
not observed as the weather was extremely bad, a 
circumstance not surprising with regard to those 
regions at that time of year. 

The eclipse of January 3, 1908, passed across the 
Pacific Ocean. Only tw^o small coral islands — Hull 
Island in the Phoenix Group, and Flint Island about 
400 miles north of Tahiti — lay in the track. Two 
expeditions set out to observe it, i.e, a combined 
American party from the Lick Observatory and the 
Smithsonian Institution of Washington, and a private 
one from England under Mr. F. K. McClean. As 
Hull Island afforded few facilities, both parties 
installed their instruments on Flint Island, although 
it was very little better. The duration of the total 
phase was fairly long — about four minutes, and the 
sun very favourably placed, being nearly overhead. 
Heavy rain and clouds, however, marred observation 
during the first minute of totality, but the remaining 
three minutes were successfully utilised, good photo- 
graphs of the corona being obtained. 

98 



Famous Eclipses of the Sun 

The next few years to come are unfortunately by 
no means favourable from the point of view of the 
eclipse observer. An eclipse will take place on June 
17, 1909; the track stretching from Greenland across 
the North Polar regions into Siberia. The geogra- 
phical situation is, however, a very awkward one, and 
totality will be extremely short — only six seconds in 
Greenland and twenty-three seconds in Siberia. 

The eclipse of May 9, 1910, will be visible in 
Tasmania. Totality will last so long as four minutes, 
but the sun will be at the time much too low in the 
sky for good observation. 

The eclipse of the following year, April 28, 191 1, 
will also be confined, roughly speaking, to the same 
quarter of the earth, the track passing across the 
old convict settlement of Norfolk Island, and then 
out into the Pacific. 

The eclipse of April 17, 191 2, will stretch from 
Portugal, through France and Belgium into North 
Germany. It will, however, be of practically no 
service to astronomy. Totality, for instance, will 
last for only three seconds in Portugal ; and, though 
Paris lies in the central track, the eclipse, which 
begins as barely total, will have changed into an 
annular one by the time it passes over that city. 

The first really favourable eclipse in the near 
future will be that of August 21, 1914. Its track will 
stretch from Greenland across Norway, Sweden, and 
Russia. This eclipse is a return, after one saros, of 
the eclipse of August 9, 1896. 

The last solar eclipse which we will touch upon 
is that predicted for June 29, 1927. It has been 
already alluded to as the first of those in the future 

99 



Astronomy of To-day 

to be total in England. The central line will stretch 
from Wales in a north-easterly direction. Stonyhurst 
Observatory, in Lancashire, will lie in the track ; but 
totality there will be very short, only about twenty 
seconds in duration. 



lOO 



CHAPTER IX 

FAMOUS ECLIPSES OF THE MOON 

The earliest lunar eclipse, of which we have any 
trustworthy information, was a total one w^hich took 
place on the 19th March, 721 B.C., and was observed 
from Babylon. For our knowledge of this eclipse 
we are indebted to Ptolemy, the astronomer, who 
copied it, along with two others, from the records of 
the reign of the Chaldean king, Merodach-Baladan. 

The next eclipse of the moon worth noting was a 
total one, which took place some three hundred years 
later, namely, in 425 B.C. This eclipse was observed 
at Athens, and is mentioned by Aristophanes in his 
play, The Clouds. 

Plutarch relates that a total eclipse of the moon, 
which occurred in 413 B.C., so greatly frightened 
Nicias, the general of the Athenians, then warring in 
Sicily, as to cause a delay in his retreat from Syracuse 
w^hich led to the destruction of his whole army. 

Seven years later — namely, in 406 B.C., the twenty- 
sixth year of the Peloponnesian War — there took place 
another total lunar eclipse of which mention is made 
by Xenophon. 

Omitting a number of other eclipses alluded to by 
ancient writers, we come to one recorded by Josephus 
as having occurred a little before the death of Herod 
the Great. It is probable that the eclipse in question 

lOI 



Astronomy of To-day 

was the total lunar one, which calculation shows to 
have taken place on the 15th September 5 B.C., and 
to have been visible in Western Asia. This is very 
important, for we are thus enabled to fix that year 
as the date of the birth of Christ, for Herod is known 
to have died in the early part of the year following 
the Nativity. 

In those accounts of total lunar eclipses, which 
have come down to us from the Dark and Middle 
Ages, the colour of the moon is nearly ahvays likened 
to '' blood." On the other hand, in an account of the 
eclipse of January 23, A.D. 753, our satellite is de- 
scribed as "covered with a horrid black shield." 
We thus have examples of the two distinct ap- 
pearances alluded to in Chapter VII., i.e. when the 
moon appears of a coppery-red colour, and when it 
is entirely darkened. 

It appears, indeed, that, in the majority of lunar 
eclipses on record, the moon has appeared of a 
ruddy, or rather of a coppery hue, and the details on 
its surface have been thus rendered visible. One of 
the best examples of a bright eclipse of this kind is 
that of the 19th March 1848, when the illumination of 
our satellite was so great that many persons could 
not believe that an eclipse was actually taking place. 
A certain Mr. Foster, who observed this eclipse from 
Bruges, states that the marki igs on the lunar disc 
were alm^ost as visible as on an "ordinary dull 
moonlight night." He goes on to say that the British 
Consul at Ghent, not knowing that there had been 
any eclipse, wrote to him for an explanation of the 
red colour of the moon on that evening. 

Out of the dark eclipses recorded, perhaps the 
102 



Famous Eclipses of the Moon 

best example is that of May i8, 1761, observed by 
Wargentin at Stockholm. On this occasion the lunar 
disc is said to have disappeared so completely, that 
it could not be discovered even with the telescope. 
Another such instance is the eclipse of June 10, 1816, 
observed from London. The summer of that year 
was particularly wet — a point worthy of notice in 
connection with the theory that these different ap- 
pearances are due to the varying state of our earth's 
atmosphere. 

Sometimes, indeed, it has happened that an eclipse 
of the moon has partaken of both appearances, part 
of the disc being visible and part invisible. An in- 
stance of this occurred in the eclipse of July 12, 
1870, when the late Rev. S. J. Johnson, one of the 
leading authorities on eclipses, who observed it, states 
that he found one-half the moon's surface quite invis- 
ible, both with the naked eye and with the telescope. 

In addition to the examples given above, there 
are three total lunar eclipses which deserve especial 
mention. 

1. A.D. 755, November 23. During the progress 
of this eclipse the moon occulted the star Aldebaran 
in the constellation of Taurus. 

2. A.D. 1493, April 2. This is the celebrated eclipse 
which is said to have so well served the purposes of 
Christopher Columbus. Certain natives having re- 
fused to supply him with provisions when in sore 
straits, he announced to them that the moon would 
be darkened as a sign of the anger of heaven. When 
the event duly came to pass, the savages were so 
terrified that they brought him provisions as much as 
he needed. 

103 



Astronomy of To-day 

3. A.D. 1610, July 6. The eclipse in question is 
notable as having been seen through the telescope, 
then a recent invention. It was without doubt the 
first so observed, but unfortunately the name of the 
observer has not come down to us. 



104 



CHAPTER X 

THE GROWTH OF OBSERVATION 

The earliest astronomical observations must have 
been made in the Dav^n of Historic Time by the men 
who tended their flocks upon the great plains. As 
they watched the clear night sky they no doubt soon 
noticed that, with the exception of the moon and 
those brilliant wandering objects known to us as the 
planets, the individual stars in the heaven remained 
apparently fixed with reference to each other. These 
seemingly changeless points of light came in time to 
be regarded as sign-posts to guide the wanderer 
across the trackless desert, or the voyager upon the 
wide sea. 

Just as when looking into the red coals of a fire, or 
when watching the clouds, our imagination conjures 
up strange and grotesque forms, so did the men of 
old see in the grouping of the stars the outlines of 
weird and curious shapes. Fed with mythological 
lore, they imagined these to be rough representations 
of ancient heroes and fabled beasts, whom they sup- 
posed to have been elevated to the heavens as a 
reward for great deeds done upon the earth. We 
know these groupings of stars to-day under the name 
of the Constellations. Looking up at them we find 
it extremely difficult to fit in the majority with the 
figures which the ancients believed them to represent. 

105 



Astronomy of To-day 

Nevertheless, astronomy has accepted the arrange- 
ment, for want of a better method of fixing the 
leading stars in the memory. 

Our early ancestors lived the greater part of their 
lives in the open air, and so came to pay more atten- 
tion in general to the heavenly orbs then we do. 
Their clock and their calendar was, so to speak, in 
the celestial vault. They regulated their hours, their 
days, and their nights by the changing positions of the 
sun, the moon, and the stars ; and recognised the 
periods of seed-time and harvest, of calm and stormy 
weather, by the rising or setting of certain well-known 
constellations. Students of the classics will recall many 
allusions to this, especially in the Odes of Horace. 

As time went on and civilisation progressed, men 
soon devised measuring instruments, by means of 
which they could note the positions of the celestial 
bodies in the sky with respect to each other ; and, 
from observations thus made, they constructed charts 
of the stars. The earliest complete survey of this 
kind, of which we have a record, is the great Cata- 
logue of stars which was made, in the second century 
B.C., by the celebrated Greek astronomer, Hipparchus, 
and in which he is said to have noted down about 
1080 stars. 

It is unnecessary to follow in detail the tedious 
progress of astronomical discovery prior to the advent 
of the telescope. Certain it is that, as time went on, 
the measuring instruments to which we have alluded 
had become greatly improved ; but, had they even 
been perfect, they would have been utterly inadequate 
to reveal those minute displacements, from which we 
have learned the actual distance of the nearest of the 

106 



The Growth of Observation 

celestial orbs. From the early times, therefore, until 
the mediaeval period of our own era, astronomy grew 
up upon a faulty basis, for the earth ever seemed so 
much the largest body in the universe, that it con- 
tinued from century to century to be regarded as the 
very centre of things. 

To the Arabians is due the credit of having kept 
alive the study of the stars during the dark ages of 
European history. They erected some fine observa- 
tories, notably in Spain and in the neighbourhood 
of Bagdad. Following them, some of the Oriental 
peoples embraced the science in earnest ; Ulugh 
Beigh, grandson of the famous Tamerlane, founding, 
for instance, a great observatory at Samarcand in 
Central Asia. The Mongol emperors of India also 
established large astronomical instruments in the chief 
cities of their empire. When the revival of learning 
took place in the West, the Europeans came to the 
front once more in science, and rapidly forged ahead 
of those who had so assiduously kept alight the lamp 
of knowledge through the long centuries. 

The dethronement of the older theories by the 
Copernican system, in which the earth was relegated 
to its true place, was fortunately soon followed by an 
invention of immense import, the invention of the 
Telescope. It is to this instrument, indeed, that we 
are indebted for our knowledge of the actual scale 
of the celestial distances. It penetrated the depths 
of space ; it brought the distant orbs so near, that 
men could note the detail on the planets, or measure 
the small changes in their positions in the sky which 
resulted from the movement of our own globe. 

It was in the year 1609 that the telescope was first 

107 



Astronomy of To-day 

constructed. A year or so previous to this a spectacle- 
maker of Middleburgh in Holland; one Hans Lipper- 
shey, had, it appears, hit upon the fact that distant 
objects, when viewed through certain glass lenses 
suitably arranged, looked nearer.^ News of this dis- 
covery reached the ears of Galileo Galilei, of Florence, 
the foremost philosopher of the day, and he at once 
applied his great scientific attainments to the con- 
struction of an instrument based upon this principle. 
The result was what was called an ^' optick tube," 
which magnified distant objects some few times. 
It was not much larger than what we nowadays 
contemptuously refer to as a ^^ spy-glass," yet its 
employment upon the leading celestial objects in- 
stantly sent astronomical science onward with a 
bound. In rapid succession Galileo announced 
world-moving discoveries ; large spots upon the face 
of the sun ; crater-like mountains upon the moon ; 
four subordinate bodies, or satellites, circling around 
the planet Jupiter ; and a strange appearance in 
connection with Saturn, which later telescopic ob- 
servers found to be a broad flat ring encircling that 
planet. And more important still, the magnified 
image of Venus showed itself in the telescope at 
certain periods in crescent and other forms ; a result 
which Copernicus is said to have announced should 
of necessity follow if his system were the true one. 

^ The principle upon which the telescope is based appears to have been 
known theoretically for a long time previous to this. The monk Roger 
Bacon, who lived in the thirteenth century* describes it very clearly ; and 
several writers of the sixteenth century have also dealt with the idea. 
Even Lippershey's claims to a practical solution of the question were hotly 
contested at the time by two of his own countrym^en, i.e. a certain Jacob 
Metius, and another spectacle-maker of Middleburgh, named Jansen. 

I08 



The Growth of Observation 

The discoveries made with the telescope produced, 
as time went on, a great alteration in the notions of 
men with regard to the imiverse at large. It must 
have been, indeed, a revelation to find that those 
points of light which they called the planets, were, 
after all, globes of a size comparable with the earth, 
and peopled perchance with sentient beings. Even 
to us, who have been accustomed since our early 
youth to such an idea, it still requires a certain 
stretch of imagination to enlarge, say, the Bright 
Star of Eve, into a body similar in size to our earth. 
The reader will perhaps recollect Tennyson's allusion 
to this in Locksley Hall, Sixty Years After : — 

" Hesper — Venus — were we native to that splendour or in Mars, 
We should see the Globe we groan in, fairest of their evening 
stars. 

"Could we dream of wars and carnage, craft and madness, lust 
and spite, 
Roaring London, raving Paris, in that point of peaceful light?" 

The form of instrument as devised by Galileo is called 
the Refracting Telescope, or '' Refractor." As we know 
it to-day it is the same in principle as his ^^optick 
tube," but it is not quite the same in construction. 
The early object-glass^ or large glass at the end, was 
a single convex lens (see Fig. 8, p. 113, ^^ Galilean") ; 
the modern one is, on the other hand, composed of 
two lenses fitted together. The attempts to construct 
large telescopes of the Galilean type met in course of 
time with a great difficulty. The magnified image of 
the object observed was not quite pure ; its edges, 
indeed, were fringed with rainbow-like colours. This 
defect was found to be aggravated with increase in 
the size of object-glasses. A method was, however, 

109 



Astronomy of To-day 

discovered of diminishing this colouration, or chromatic 
aberration as it is called from the Greek word %pwyua 
(chroma)^ which means colour, viz. by making tele- 
scopes of great length and only a few inches in width. 
But the remedy was, in a way, worse than the disease ; 
for telescopes thus became of such huge proportions 
as to be too unwieldy for use. Attempts were made 
to evade this unwieldiness by constructing them with 
skeleton tubes (see Plate II., p. no), or, indeed, even 
without tubes at all ; the object-glass in the tubeless 
or ^'aerial" telescope being fixed at the top of a high 
post, and the eye-piece^ that small lens or combination 
of lenses, which the eye looks directly into, being kept 
in line with it by means of a string and manoeuvred 
about near the ground (Plate IIL, p. 112). The idea 
of a telescope without a tube may appear a contradic- 
tion in terms ; but it is not really so, for the tube 
adds nothing to the magnifying power of the instru- 
ment, and is, in fact, no more than a m.ere device for 
keeping the object-glass and eye-piece in a straight 
line, and for preventing the observer from being 
hindered by stray lights in his neighbourhood. It 
goes without saying, of course, that the image of a 
celestial object will be more clear and defined when 
examined in the darkness of a tube. 

The ancients, though they knew nothing of tele- 
scopes, had, however, found out the merit of a tube 
in this respect ; for they employed simple tubes, 
blackened on the inside, in order to obtain a clearer 
view of distant objects. It is said that Julius Caesar, 
before crossing the Channel, surveyed the opposite 
coast of Britain through a tube of this kind. 

A few of the most famous of the immensely long 
no 



The Growth of Observation 

telescopes above alluded to are worthy of mention. 
One of these, 123 feet in length, was presented to the 
Royal Society of London by the Dutch astronomer 
Huyghens. Hevelius of Danzig constructed a skele- 
ton one of 150 feet in length (see Plate II., p. no). 
Bradley used a tubeless one 212 feet long to measure 
the diameter of Venus in 1722 ; while one of 600 feet 
is said to have been constructed, but to have proved 
quite unworkable ! 

Such difficulties, however, produced their natural 
result. They set men at work to devise another kind 
of telescope. In the new form, called the Reflecting 
Telescope, or ^' Reflector," the light coming from the 
object under observation was reflected into the eye-piece 
from the surface of a highly polished concave metallic 
mirror, or speculum^ as it was called. It is to Sir 
Isaac Newton that the world is indebted for the re- 
flecting telescope in its best form. That philosopher 
had set himself to investigate the causes of the rainbow- 
like, or prismatic colours which for a long time had 
been such a source of annoyance to telescopic ob- 
servers ; and he pointed out that, as the colours were 
produced in the passage of the rays of light through 
the glass, they would be entirely absent if the light 
were reflected from the surface of a mirror instead. 

The reflecting telescope, however, had in turn cer- 
tain drawbacks of its own. A mirror, for instance, 
can plainly never be polished to such a high degree 
as to reflect as much light as a piece of transparent 
glass will let through. Further, the position of the 
eye-piece is by no means so convenient. It cannot, 
of course, be pointed directly towards the mirror, for 
the observer would then have to place his head right 

III 



Astronomy of To-day 

in the way of the Hght coming from the celestial 
object, and would thus, of course, cut it off. In order 
to obviate this difficulty^ the following device was 
employed by Newton in his telescope, of which he 
constructed his first example in 1668. A small, flat 
mirror was fixed by thin wires in the centre of the 
tube of the telescope, and near to its open end. It 
was set slant-wise, so that it reflected the rays of light 
directly into the eye-piece, which was screwed into a 
hole at the side of the tube (see Fig. 8, p. 113, "New- 
tonian "). 

Although the Newtonian form of telescope had the 
immense advantage of doing away with the prismatic 
colours, yet it wasted a great deal of light ; for the 
objection in this respect with regard to loss of light 
by reflection from the large mirror applied, of course, 
to the small mirror also. In addition, the position 
of the "flat," as the small mirror is called, had the 
further effect of excluding from the great mirror a 
certain proportion of light. But the reflector had 
the advantage, on the other hand, of costing less to 
m_ake than the refractor, as it was not necessary to 
procure flawless glass for the purpose. A disc of a 
certain metallic composition, an alloy of copper and 
tin, known in consequence as speculum metaly had 
merely to be cast ; and this had to be ground and 
polished upon one side only, whereas a lens has to be 
thus treated upon both its sides. It was, therefore, pos- 
sible to make a much larger instrument at a great 
deal less labour and expense. 

We have given the Newtonian form as an example 
of the principle of the reflecting telescope. A some- 
what similar instrument had, however, been projected, 

112 




Plate III. A Tubeless, or "Aerial" Telescope 

From an illustration in the Opera Varia of Christian Huyghens. 



(Page no) 



The Growth of Observation 

though not actually constructed, by James Gregory 
a few years earlier than Newton's, i.e, in 1663. In 

/iefracitn^ Telescopes ^ 



Galilean 



Achromatic 



fiefiectin^ Telescopes 



f^' 



'M% 



A ObjeclGlass 
B.Eyepicce 



4 



¥} 



Aleirtonfan, 



/ferscAellan 



^ 



Gregorian 



... 1^ 



] 



A Speculum, mijwr 
B. Eyepiece 
C.flal 



A, Speculum mlrwr 

B. Eyepiece 



i^^A. Speculupt mirror 

B. Eyepiece 

C. Small CcMove mmr 



'\{_B A. Speculum nilnvr 
.!pi3l^ B. Eyepiece 
J] 



C. Small Convermiiivr 



Cassegraniaa 

Fig. 8. — The various types of Telescope. All the above telescopes are 
pointed in the same direction ; that is to say, the rays of light from 
the object are coming from the left-hand side. 

this form of reflector, known as the ''Gregorian" 
telescope, a hole was made in the big concave mirror ; 
and a small mirror,- also concave, which faced it at a 

113 H 



Astronomy of To-day 

certain distance, received the reflected rays, and re- 
flected them back again through the hole in question 
into the eye-piece, which was fixed just behind (see 
Fig. 8, p. 113, '^Gregorian "). The Gregorian had thus 
the sentimental advantage of being pointed directly at 
the object. The hole in the big mirror did not cause 
any loss of light, for the central portion in which it 
was made was anyway unable to receive light through 
the small mirror being directly in front of it. An 
adaptation of the Gregorian was the " Cassegrainian " 
telescope, devised byCassegrain in 1672, which'differed 
from it chiefly in the small mirror being convex instead 
of concave (see Fig. 8, p. 113, ^^ Cassegrainian "). These 
direct-view forms of the reflecting telescope were much 
in vogue about the middle of the eighteenth century, 
when many beautiful examples of Gregorians were made 
by the famous optician, James Short, of Edinburgh. 

An adaptation of the Newtonian type of telescope 
is known as the '^ Herschelian," from being the kind 
favoured by Sir William Herschel. It is, however, 
only suitable in immense instruments, such as Hers- 
chel was in the habit of employing. In this form 
the object-glass is set at a slight slant, so that the 
light coming from the object is reflected straight into 
the eye-piece, which is fixed facing it in the side of 
the tube (see Fig. 8, p. 113, "Herschelian"). This 
telescope has an advantage over the other forms of 
reflector through the saving of light consequent on 
doing away with the second reflection. There is, how- 
ever, the objection that the slant of the object-glass 
is productive of some distortion in the appearance 
of the object observed ; but this slant is of necessity 
slight when the length of the telescope is very great, 

114 



The Growth of Observation 

The principle of this type of telescope had been 
described to the French Academy of Sciences as early 
as 1728 by Le Maire, but no one availed himself of 
the idea until 1776, when Herschel tried it. At first, 
however, he rejected it ; but in 1786 he seems to have 
found that it suited the huge instruments which he 
was then making. Herschel's largest telescope, con- 
structed in 1789, was about four feet in diameter and 
forty feet in length. It is generally spoken of as the 
^' Forty-foot Telescope," though all other instruments 
have been known by their diameters^ rather than by 
their lengths. 

To return to the refracting telescope. A solution 
of the colour difficulty was arrived at in 1729 (two 
years after Newton's death) by an Essex gentleman 
named Chester Moor Hall. He discovered that by 
making a double object-glass, composed of an outer 
convex lens and an inner concave lens, made respec- 
tively of different kinds of glass, i.e, crown glass and 
flint glass, the troublesome colour effects could be, 
to a very great extent^ removed. Hall's investigations 
appear to have been rather of an academic nature ; 
and, although he is believed to have constructed a 
small telescope upon these lines, yet he seems to 
have kept the matter so much to himself that it was 
not until the year 1758 that the first example of the 
new instrument was given to the world. This was 
done by John DoUond, founder of the well-known 
optical firm of Dollond, of Ludgate Hill, London, who 
had, quite independently, re-discovered the principle. 

This ^'Achromatic" telescope, or telescope ''free 
from colour effects," is the kind ordinarily in use at 
present, whether for astronomical or for terrestrial 

115 



Astronomy of To-day 

purposes (see Fig. 8, p. 113, ^^ Achromatic "). The ex- 
pense of making large instruments of this type is very 
great, for, in the object-glass alone, no less than four 
surfaces have to be ground and polished to the re- 
quired curves ; and, usually, the two lenses of which 
it is composed have to fit quite close together. 

With the object of evading the expense referred 
to, and of securing complete freedom from colour 
effects, telescopes have even been made, the object- 
glasses of which w^ere composed of various transparent 
liquids placed between thin lenses ; but leakages, and 
currents set up within them by changes of tempera- 
ture, have defeated the ingenuity of those who devised 
these substitutes. 

The solution of the colour difficulty by means of 
Dollond's achromatic refractor has not, however, 
ousted the reflecting telescope in its best, or New- 
tonian form, for which great concave mirrors made 
of glass, covered wdth a thin coating of silver and 
highly polished, have been used since about 1870 
instead of metal mirrors. They are very much lighter 
in w^eight and cheaper to make than the old specula ; 
and though the silvering, needless to say, deteriorates 
with time, it can be renew^ed at a comparatively trifling 
cost. Also these mirrors reflect much more light, 
and give a clearer view, than did the old metallic ones. 

When an object is viewed through the type of 
astronomical telescope ordinarily in use, it is seen 
upside down. This is, however, a matter of very 
small moment in dealing with celestial objects ; for, 
as they are usually round, it is really not of much 
consequence w^hich part w^e regard as top and w^hich 
as bottom. Such an inversion would, of course, be 

116 



The Growth of Observation 

most inconvenient when viewing terrestrial objects. 
In order to observe the latter we therefore employ 
what is called a terrestrial telescope, which is merely 
a refractor with some extra lenses added in the eye 
portion for the purpose of turning the inverted image 
the right way up again. These extra lenses, needless 
to say, absorb a certain amount of light ; wherefore 
it is better in astronomical observation to save 
light by doing away with them, and putting up 
with the slight inconvenience of seeing the object 
inverted. 

This inversion of images by the astronomical 
telescope must be specially borne in mind with regard 
to the photographs of the moon in Chapter XVI. 

In the year 1825 the largest achromatic refractor 
in existence was one of nine and a half inches in 
diameter constructed by Fraunhofer for the Observa- 
tory of Dorpat in Russia. The largest refractors in 
the world to-day are in the United States, Le. the forty- 
inch of the Yerkes Observatory (see Plate IV., p. 118), 
and the thirty-six inch of the Lick. The object- 
glasses of these and of the thirty-inch telescope of 
the Observatory of Pulkowa, in Russia, were made 
by the great optical house of Alvan Clark & Sons, 
of Cambridge, Massachusetts, U.S.A. The tubes and 
other portions of the Yerkes and Lick telescopes 
were, however, constructed by the Warner and 
Swasey Co., of Cleveland, Ohio. 

The largest reflector, and so the largest telescope 
in the world, is still the six-foot erected by the late 
Lord Rosse at Parsonstown in Ireland, and completed 
in the year 1845. It is about fifty-six feet in length. 
Next come two of five feet, with mirrors of silver on 

117 



Astronomy of To-day 

glass ; one of them made by the late Dr. Common, 
of Ealing, and the other by the American astronomer, 
Professor G. W. Ritchey. The latter of these is in- 
stalled in the Solar Observatory belonging to Carnegie 
Institution of Washington, which is situated on Mount 
Wilson in California. The former is now at the 
Harvard College Observatory, and is considered by 
Professor Moulton to be probably the most efficient 
reflector in use at present. Another large reflector 
is the three-foot made by Dr. Common. It came into 
the possession of Mr. Crossley of Halifax, who pre- 
sented it to the Lick Observatory, where it is now 
known as the ^' Crossley Reflector." 

Although to the house of Clark belongs, as we have 
seen, the credit of constructing the object-glasses of 
the largest refracting telescopes of our time, it has 
nevertheless keen competitors in Sir Howard Grubb, 
of Dublin, and such well-known firms as Cooke of 
York and Steinheil of Munich. In the four-foot 
reflector, made in 1870 for the Observatory of 
Melbourne by the firm of Grubb, the Cassegrainian 
principle was employed. 

With regard to the various merits of refractors and 
reflectors much might be said. Each kind of instru- 
ment has, indeed, its special advantages; though 
perhaps, on the whole, the most perfect type of 
telescope is the achromatic refractor. 

In connection with telescopes certain devices have 
from time to time been introduced, but these merely 
aim at the convenience of the observer and do not 
supplant the broad principles upon which are based 
the various types of instrument above described. 
Such, for instance, are the '^ Siderostat," and another 

118 




Plate IV. The Great Verkes Telescoi^e 

Great telescope at the Yerkes Observatory of the University of Chicago, Williams Bay, 
Wisconsin,_U.S.x'\. It was erected in 1896-7, and is the largest refracting telescope in the 
world. Diameter of object-glass, 40 inches ; length of telescope, about 60 feet. The 
object-glass was made by the firm of Alvan Clark and Sons, of Cambridge, Massachusetts ; 
the other portions of the instalment by the Warner and Swasey Co., of Cleveland. Ohio. 

(Page 117') 



The Growth of Observation 

form of it called the ''Coelostat/' in which a plane 
mirror is made to revolve in a certain manner, so 
as to reflect those portions of the sky which are 
to be observed, into the tube of a telescope kept 
fixed. Such too are the '^ Equatorial Coude" of the 
late M. Loewy, Director of the Paris Observatory, 
and the " Sheepshanks Telescope " of the Observatory 
of Cambridge, in which a telescope is separated 
into two portions, the eye-piece portion being fixed 
upon a downward slant, and the object-glass portion 
jointed to it at an angle and pointed up at the sky. 
In these two instruments (which, by the way, differ 
materially) an arrangement of slanting mirrors in the 
tubes directs the journey of the rays of light from the 
object-glass to the eye-piece. The observer can thus 
sit at the eye-end of his telescope in the warmth and 
comfort of his room, and observe the stars in the 
same unconstrained manner as if he were merely 
looking down into a microscope. 

Needless to say, devices such as these are subject 
to the drawback that the mirrors employed sap a 
certain proportion of the rays of light. It will be 
remembered that we made allusion to loss of light 
in this way, when pointing out the advantage in light 
grasp of the Herschelian form of telescope, where 
only one reflection takes place, over the Newtonian 
in which there are two. 

It is an interesting question as to whether tele- 
scopes can be made much larger. The American 
astronomer, Professor G. E. Hale, concludes that the 
limit of refractors is about five feet in diameter, but 
he thinks that reflectors as large as nine feet in 
diameter might now be made. As regards refractors 

119 



Astronomy of To-day 

there are several strong reasons against augmenting 
their proportions. First of all comes the great cost. 
Secondly, since the lenses are held in position merely 
round their rims, they will bend by their weight in 
the centres if they are made much larger. On the 
other hand; attempts to obviate this, by making the 
lenses thicker, would cause a decrease in the amount 
of light let through. 

But perhaps the greatest stumbling-block to the 
construction of larger telescopes is the fact that the 
unsteadiness of the air will be increasingly magnified. 
And further, the larger the tubes become, the more 
difficult will it be to keep the air within them at one 
constant temperature throughout their lengths. 

It would, indeed, seem as if telescopes are not 
destined greatly to increase in size, but that the means 
of observation will break out in some new direction, 
as it has already done in the case of photography 
and the spectroscope. The direct use of the eye 
is gradually giving place to indirect methods. We 
are, in fact, now feeling rather than seeing our way 
about the universe. Up to the present, for instance, 
we have not the slightest proof that life exists else- 
where than upon our earth. But who shall say 
that the twentieth century has not that in store for 
us, by which the presence of life in other orbs may 
be perceived through some form of vibration trans- 
mitted across illimitable space ? There is no use 
speaking of the impossible or the inconceivable. 
After the extraordinary revelations of the spectroscope 
— nay, after the astounding discovery of Rontgen — 
the word impossible should be cast aside, and incon- 
ceivability cease to be regarded as any criterion. 

1 20 



CHAPTER XI 

SPECTRUM ANALYSIS 

If white light (that of the sun, for instance) be passed 
through a glass prism, namely, a piece of glass of 
triangular shape, it will issue from it in rainbow-tinted 
colours. It is a common experience with any of us 
to notice this when the sunlight shines through cut- 
glass, as in the pendant of a chandelier, or in the 
stopper of a wine-decanter. 

The same effect may be produced when light 
passes through water. The Rainbow, which we all 
know so well, is merely the result of the sunlight 
passing through drops of falling rain. 

White light is composed of rays of various colours. 
Red, orange, yellow, green, blue, indigo, and violet, 
taken all together, go, in fact, to make up that effect 
which we call white. 

It is in the course of the refraction, or bending of 
a beam of light, when it passes in certain conditions 
through a transparent and denser medium, such as 
glass or water, that the constituent rays are sorted 
out and spread in a row according to their various 
colours. This production of colour takes place 
usually near the edges of a lens ; and, as will be 
recollected, proved very obnoxious to the users of 
the old form of refracting telescope. 

It is, indeed, a strange irony of fate that this very 

121 



Astronomy of To-day 

same production of colour, which so hindered as- 
tronomy in the past, should have aided it in recent 
years to a remarkable degree. If sunlight, for instance, 
be admitted through a narrow slit before it falls upon 
a glass prism, it will issue from the latter in the form 
of a band of variegated colour, each colour blending 
insensibly with the next. The colours arrange them- 
selves always in the order which we have mentioned. 
This seeming band is, in reality, an array of countless 
coloured images of the original slit ranged side by 
side ; the colour of each image being the slightest 
possible shade different from that next to it. This 
strip of colour when produced by sunlight is called 
the ^^ Solar Spectrum" (see Fig. 9, p. 123). A similar 
strip, or spectrum^ will be produced by any other light ; 
but the appearance of the strip, with regard to pre- 
ponderance of particular colours, will depend upon 
the character of that light. Electric light and gas 
light yield spectra not unlike that of sunlight ; but 
that of gas is less rich in blue and violet than that of 
the sun. 

The Spectroscope, an instrument devised for the 
examination of spectra, is, in its simplest form, com- 
posed of a small tube with a narrow slit and prism 
at one end, and an eye-piece at the other. If we drop 
ordinary table salt into the flame of a gas light, the 
flame becomes strongly yellow. If, then, we observe 
this yellow flame with the spectroscope, we find that 
its spectrum consists almost entirely of two bright 
yellow transverse lines. Chemically considered ordi- 
nary table salt is sodium chloride ; that is to say, a 
compound of the metal sodium and the gas chlorine. 
Now if other compounds of sodium be experimented 

122 



Speetrum Analysis 






t 

I 






a. 

'o 
m 
a 

H 
I 

o 



^ga^^^:^5HgO 



123 



Astronomy of To-day 

with in the same manner, it will soon be found that 
these two yellow lines are characteristic of sodium 
when turned into vapour by great heat. In the same 
manner it can be ascertained that every element, 
when heated to a condition of vapour, gives as its 
spectrum a set of lines peculiar to itself. Thus the 
spectroscope enables us to find out the composition 
of substances by reducing them to vapour in the 
laboratory. 

In order to increase the power of a spectroscope, 
it is necessary to add to the number of prisms. Each, 
extra prism has the effect of lengthening the coloured 
strip still more, so that lines, which at first appeared 
to be single merely through being crowded together, 
are eventually drawn apart and become separately 
distinguishable. 

On this principle it has gradually been determined 
that the sun is composed of elements similar to those 
which go to make up our earth. Further, the com- 
position of the stars can be ascertained in the same 
manner ; and we find them formed on a like pattern, 
though with certain elements in greater or less pro- 
portion as the case may be. It is in consequence 
of our thus definitely ascertaining that the stars are 
self-luminous, and of a sun-like character, that we are 
enabled to speak of them as suns, or to call the sun 
a star. 

In endeavouring to discover the elements of which 
the planets and satellites of our system are composed, 
we, however, find ourselves baffled, for the simple 
reason that these bodies emit no real light of their 
own. The light which reaches us from them, being 
merely reflected sunlight, gives only the ordinary 

124 



I 



Spectrum Analjrsis 



solar spectrum when examined with the spectroscope. 
But in certain cases we find that the solar spectrum 
thus^ viewed shows traces of being weakened, or rather 
of suffering absorption ; and it is concluded that this 
may be due to the sunlight having had to pass through 
an atmosphere on its way to and from the surface 
of the planet from which it is reflected to us. 

Since the sun is found to be composed of elements 
similar to those which go to make up our earth, we 
need not be disheartened at this failure of the spec- 
troscope to inform us of the composition of the 
planets and satellites. We are justified, indeed, in 
assuming that more or less the same constituents 
run through our solar system ; and that the elements 
of which these bodies are composed are similar to 
those which are found upon our earth and in the 
sun. 

The spectroscope supplies us with even more in- 
formation. It tells us, indeed, whether the sunlike 
body which we are observing is moving away from us 
./or towards us. A certain slight shifting of the lines 
towards the red or violet end of the spectrum re- 
spectively, is found to follow such movement. This 
method of observation is known by the name of 
Doppler's Method^ and by it we are enabled to confirm 
the evidence which the sunspots give us of the rota- 
tion of the sun ; for we find thus that one edge 

^ The idea, initiated by Cliristian Doppler at Prague in 1842, was 
originally applied to sound. The approach or recession of a source from 
which sound is coming is invariably accompanied by alterations of pitch, 
as the reader has no doubt noticed when a whistling railway-engine has 
approached him or receded from him. It is to Sir William Huggins, 
however, that we are indebted for the application of the principle to 
spectroscopy. This he gave experimental proof of in the year 1S68. 



Astronomy of To-day 

of that body is continually approaching us, and the 
other edge is continually receding from us. Also, we 
can ascertain in the same manner that certain of the 
stars are moving towards us, and certain of them 
away from us. 



126 



CHAPTER XII 

THE SUN 

The sun is the chief member of our system. It 
controls the motions of the planets by its immense 
gravitative power. Besides this it is the most im- 
portant body in the entire universe, so far as we are 
concerned ; for it pours out continually that flood of 
light and heat, without which life, as we know it, 
would quickly become extinct upon our globe. 

Light and heat, though not precisely the same thing, 
may be regarded, however, as next-door neighbours. 
The light rays are those which directly affect the 
eye and are comprised in the visible spectrum. We 
feel the heat rays, the chief of which are beyond the 
red portion of the spectrum. They may be investi- 
gated with the bolometer^ an instrument invented by 
the late Professor Langley. Chemical rays — for in- 
stance, those radiations which affect the photographic 
plate — are for the most part also outside the visible 
spectrum. They are, however, at the other end of it, 
namely, beyond the violet. 

Such a scale of radiations may be compared to the 
keyboard of an imaginary piano, the sound from only 
one of whose octaves is audible to us. 

The brightest light we know on the earth is dull 
compared with the light of the sun. It would, indeed, 
look quite dark if held up against it. 

127 



Astronomy of To-day 

It is extremely difficult to arrive at a precise notion 
of the temperature of the body of the sun. However, 
it is far in excess of any temperature which we can 
obtain here, even in the most powerful electric fur- 
nace. 

A rough idea of the solar heat may be gathered 
from the calculation that if the sun's surface w^ere 
coated all over with a layer of ice 4000 feet thick, 
it would melt through this completely in one hour. 

The sun cannot be a hot body merely cooling ; for 
the rate at which it is at present giving off heat could 
not in such circumstances be kept up, according to 
Professor Moulton, for more than 3000 years. Further, 
it is not a mere burning mass, like a coal fire, for 
instance ; as in that case about a thousand years 
w^ould show a certain drop in temperature. No per- 
ceptible diminution of solar heat having taken place 
within historic experience, so far as can be ascer- 
tained, we are driven to seek some more abstruse 
explanation. 

The theory which seems to have received most 
acceptance is that put forward by Helmholtz in 1854. 
His idea was that gravitation produces continual con- 
traction, or falling in of the outer parts of the sun ; 
and that this falling in, in its turn, generates enough 
heat to compensate for what is being given off. The 
calculations of Helmholtz showed that a contraction 
of about 100 feet a year from the surface towards 
the centre would suffice for the purpose. In recent 
years, however, this estimate has been extended to 
about 180 feet. Nevertheless, even with this increased 
figure, the shrinkage required is so slight in com- 
parison with the immense girth of the sun, that it 

128 



The Sun 

would take a continual contraction at this rate for 
about 6000 years, to show even in our finest tele- 
scopes that any change in the size of that body was 
taking place at all. Upon this assumption of con- 
tinuous contraction, a time should, however, eventually 
be reached when the sun will have shrunk to such a 
degree of solidity, that it will not be able to shrink 
any further. Then, the loss of heat not being made 
up for any longer, the body of the sun should begin 
to m^ow cold. But we need not be distressed on this 
account ; for it will take some 10,000,000 years, 
according to the above theory, before the solar orb 
becomes too cold to support life upon our earth. 

Since the discovery of radium it has, on the other 
hand, been suggested, and not unreasonably, that 
radio-active matter may possibly play an important 
part in keeping up the heat of the sun. But the body 
of scientific opinion appears to consider the theory of 
contraction as a result of gravitation, which has been 
outlined above, to be of itself quite a sound explana- 
tion. Indeed, the late Lord Kelvin is said to have 
held to the last that it was amply sufficient to account 
for the underground heat of the earth, the heat of the 
sun, and that of all the stars in the universe. 

One great difficulty in forming theories with regard 
to the sun, is the fact that the temperature and gravi- 
tation there are enormously in excess of anything \\q 
meet with upon our earth. The force of gravity at 
the sun's surface is, indeed, about twenty-seven times 
that at the surface of our globe. 

The earth's atmosphere appears to absorb about 
one-half of the radiations which come to us from the 
sun. This absorptive effect is very noticeable when 

129 I 



Astronomy of To-day 

the solar orb is low down in our sky, for iits light and 
heat are then clearly much reduced. Of the light 
rays, the blue ones are the most easily absorbed in 
this way ; which explains why the sun looks red 
when near the horizon. It has then, of course, to 
shine through a much greater thickness of atmosphere 
than when high up in the heavens. 

What astonishes one most about the solar radiation, 
is the immense amount of it that is apparently wasted 
into space in comparison with what falls directly 
upon the bodies of the solar system. Only about the 
one-hundred-millionth is caught by all the planets 
together. What becomes of the rest we cannot tell. 

That brilliant white body of the sun, which we see, 
is enveloped by several layers of gases and vaporous 
matter, in the same manner as our globe is enveloped 
by its atmosphere (see Fig. lo, p. 131). These are 
transparent, just as our atmosphere is transparent ; 
and so we see the white bright body of the sun right 
through them. 

This white bright portion is called the Photosphere, 
From it comes most of that light and heat which we 
see and feel. We do not know what lies under the 
photosphere, but, no doubt, the more solid portions of 
the sun are there situated. Just above the photosphere, 
and lying close upon it, is a veil of smoke-like haze. 

Next upon this is what is known as the Reversing 
Layer, which is between 500 and 1000 miles in thick- 
ness. It is cooler than the underlying photosphere, 
and is composed of glowing gases. Many of, the 
elements which go to make up our earth are present 
in the reversing layer in the form of vapour. 

The Chromosphere^ of which especial mention has 

130 



The Sun 

already been made in dealing with eclipses of the 
sun, is another layer lying immediately upon the last 
one. It is between 5000 and 10,000 miles in thickness. 
Like the reversing layer, it is composed of glowing 
gaseS; chief among which is the vapour of hydrogen. 
The colour of the chromosphere is, in reality, a 




tos frh^re 



Fig. 10. — A section through the Sun, showing how the 
prominences rise from the chromosphere. 



brilliant scarlet ; but, as we have already said, the 
intensely white light of the photosphere shines through 
it from behind, and entirely overpowers its redness. 
The upper portion of the chromosphere is in violent 
agitation, like the waves of a stormy sea, and from 
it rise those red prominences which, it will be re- 
collected, are such a notable feature in total solar 
eclipses. 

131 



Astronomy of To=day 

The Corona lies next in order outside the chromo- 
sphere, and is, so far as we know, the outermost of 
the accompaniments of the sun. This halo of pearly- 
white light is irregular in outline, and fades away 
into the surrounding sky. It extends outwards from 
the sun to several millions of miles. As has been 
stated, we can never see the corona unless, when 
during a total solar eclipse, the moon has, for the 
time being, hidden the brilliant photosphere com- 
pletely from our view. 

The solar spectrum is really composed of three 
separate spectra commingled, i.e. those of the photo- 
sphere, of the reversing layer, and of the chromo- 
sphere respectively. 

If, therefore, the photosphere could be entirely 
removed, or covered up, we should see only the 
spectra of those layers which lie upon it. Such a 
state of things actually occurs in a total eclipse of 
the sun. When the moon's body has crept across 
the solar disc, and hidden the last piece of photo- 
sphere, the solar spectrum suddenly becomes what 
is technically called ^'reversed," — the dark lines cross- 
ing it changing into bright lines. This occurs because 
a strip of those layers which lie immediately upon 
the photosphere remains still uncovered. The lower 
of these layers has therefore been called the ^^re- 
versing layer," for want of a better name. After a 
second or two this reversed spectrum mostly vanishes, 
and an altered spectrum is left to view. Taking into 
consideration the rate at which the moon is moving 
across the face of the sun, and the very short time 
during which the spectrum of the reversing layer 
lasts, the thickness of that layer is estimated to be 

132 



The Sun 

not more than a few hundred miles. In the same 
way the last of the three spectra — namely, that of 
the chromosphere — remains visible for such a time as 
allows us to estimate its depth at about ten times that 
of the reversing layer, or several thousand miles. 

When the chromosphere, in its turn during a total 
eclipse, has been covered by the moon, the corona 
alone is left. This has a distinct spectrum of its own 
also ; wherein is seen a strange line in the green 
portion, which does not tally with that of any element 
we are acquainted with upon the earth. This un- 
known element has received for the time being the 
name of '< Coronium." 



133 



CHAPTER XIII 

THE SUN—conf'muecI 

The various parts of the Sun will now be treated of 
in detail. 

I. Photosphere. 

The photosphere, or " light-sphere/' from the Greek 
<l>m (p^os), which means lz£-kt, is, as we have already 
said, the innermost portion of the sun which can be 
seen. Examined through a good telescope it shows a 
finely mottled structure, as of brilliant granules, some- 
what like rice grains, with small dark spaces lying in 
between them. It has been supposed that we have 
here the process of some system of circulation by 
which the sun keeps sending forth its radiations. In 
the bright granules we perhaps see masses of intensely 
heated matter, rising from the interior of the sun. 
The dark interspaces may represent matter which has 
become cooled and darkened through having parted 
with its heat and light, and is falling back again into 
the solar furnace. 

The sun spots, so familiar to every one nowadays, are 
dark patches which are often seen to break out in the 
photosphere (see Plate V., p. 134). They last during 
various periods of time ; sometimes only for a few days, 
sometimes so long as a month or more. A spot is usually 
composed of a dark central, portion called the umbra^ 

134 



Plate V. The Sun, showing several groups of Spots 

From a photograph taken at the Royal Observatory, Greenwich. The cross-lines seen 
on the disc are in no way connected with the Sun, but belong to the telescope through 
which the photograph was taken. 



(Page 134) 



The Sun 

and a less dark fringe around this called Wx^ penumbra 
(see Plate VI., p. 136). The umbra ordinarily has the 
appearance of a deep hole in the photosphere ; but, 
that it is a hole at all, has by no means been 
definitely proved. 

Sun spots are, as a rule, some thousands of miles 
across. The umbra of a good-sized spot could indeed 
engulf at once many bodies the size of our earth. 

Sun spots do not usually appear singly, but in 
groups. The total area of a group of this kind may 
be of immense extent ; even so great as to cover the 
one-hundredth part of the whole surface of the sun. 
Very large spots, when such are present, may be seen 
without any telescope ; either through a piece of 
smoked glass, or merely with the naked eye when the 
air is misty, or the sun low on the horizon. 

The umbra of a spot is not actually dark. It only 
appears so in contrast with the brilliant photosphere 
around. 

Spots form, grow to a large size in comparatively 
short periods of time, and then quickly disappear. 
They seem to shrink away as a consequence of the 
photosphere closing in upon them. 

That the sun is rotating upon an axis, is shown by 
the continual change of position of all spots in one 
constant direction across his disc. The time in which 
a spot is carried completely round depends, however, 
upon the position which it occupies upon the sun's 
surface. A spot situated near the equator of the sun 
goes round once in about twenty-five days. The 
further a spot is situated from this equator, the longer 
it takes. About twenty-seven days is the time taken 
by a spot situated midway between the equator and 

135 



Astronomy of To-day 

the solar poles. Spots occur to the north of the sun's 
equator, as well as to the south ; though, since regular 
observations have been made — that is to say, during 
the past fifty years or so — they appear to have broken 
out a little more frequently in the southern parts. 

From these considerations it will be seen that the 
sun does not rotate as the earth does, but that 
different portions appear to move at different speeds. 
Whether in the neighbourhood of the solar poles the 
time of rotation exceeds twenty-seven days we are 
unable to ascertain, for spots are not seen in those 
regions. No explanation has yet been given of this 
peculiar rotation ; and the most we can say on the 
subject is that the sun is not by any means a solid 
body. 

Faculcs (Latin, little torches) are brilliant patches 
which appear here and there upon the sun's surface, 
and are in some way associated with spots. Their 
displacement, too, across the solar face confirms the 
evidence which the spots give us of the sun's rotation. 

Our proofs of this rotation are still further strength- 
ened by the Doppler spectroscopic method of obser- 
vation alluded to in Chapter XL As was then stated, 
one edge of the sun is thus found to be continually 
approaching us, and the other side continually re- 
ceding from us. The varying rates of rotation, which 
the spots and faculse give us, are duly confirmed by 
this method. 

The first attempt to bring some regularity into 
the question of sun-spots was the discovery by 
Schwabe, in 1852, that they were subject to a regular 
variation. As a matter of fact they wax and wane in 
their number, and the total area which they cover, in 

136 




Plate VI. Photograph ok a Sunspot 

This fine picture was taken by the late M. Janssen. The granular structure of the Sun' 
surface is here well represented. (From A'/io-a'/i-cft^w) 

(Page 135) 



The Sun 

the course of a period, or cycle, of on an average 
about II J years; being at one part of this period 
large and abundant, and at another few and small. 
This period of iij years is known as the sun spot 
cycle. No explanation has yet been given of the 
curious round of change, but the period in question 
seems to govern most of the phenomena connected 
with the sun. 

II. Reversing Layer. 

This is a layer of relatively cool gases lying im- 
mediately upon the photosphere. We never see it 
directly ; and the only proof we have of its presence 
is that remarkable reversal of the spectrum already 
described, when during an instant or two in a total 
eclipse, the advancing edge of the moon, having just 
hidden the brilliant photosphere, is moving across 
the fine strip which the layer then presents edge-wise 
towards us. The fleeting moments during which 
this reversed spectrum lasts, informs us that the layer 
is comparatively shallow ; little more indeed than 
about 500 miles in depth. 

The spectrum of the reversing layer, or *^ flash 
spectrum," as it is sometimes called on account of the 
instantaneous character with which the change takes 
place, was, as we have seen, first noticed by Young 
in 1870 ; and has been successfully photographed 
since then during several eclipses. The layer itself 
appears to be in a fairly quiescent state ; a marked 
contrast to the seething photosphere beneath, and 
the agitated chromosphere above. 



137 



Astronomy of To-day 



III. The Chromosphere. 

The Chromosphere— so called from the Greek 
^/owyLta {chroma)y which signifies colour — is a layer of 
gases lying immediately upon the preceding one. Its 
thickness is, however, plainly much the greater of the 
two ; for whereas the reversing layer is only revealed 
to us indirectly by the spectroscope, a portion of the 
chromosphere may clearly be seen in a total eclipse 
in the form of a strip of scarlet light. The time 
which the moon's edge takes to traverse it tells us 
that it must be about ten times as deep as the revers- 
ing layer, namely, from 5000 to 10,000 miles in 
depth. Its spectrum shows that it is composed 
chiefly of hydrogen, calcium and helium, in the state 
of vapour. Its red colour is mainly due to glowing 
hydrogen. The element helium, which it also 
contains, has received its appellation from r]kio<^ 
{kelios), the Greek name for the sun ; because, at the 
time when it first attracted attention, there appeared 
to be no element corresponding to it upon our earth, 
and it was consequently imagined to be confined to 
the sun alone. Sir William Ramsay, however, dis- 
covered it to be also a terrestrial element in 1895, and 
since then it has come into much prominence as one 
of the products given off by radium. 

Taking into consideration the excessive force of 
gravity on the sun, one would expect to find the 
chromosphere and reversing layer growing gradually 
thicker in the direction of the photosphere. This, 
however, is not the case. Both these layers are 
strangely enough of the same densities all through ; 

138 



The Sun 

which makes it suspected that, in these regionS; the 
force of gravity may be counteracted by some other 
force or forces, exerting a powerful pressure out- 
wards from the sun. 



IV. The Prominences. 

We have already seen, in dealing with total 
eclipses, that the exterior surface of the chromo- 
sphere is agitated like a stormy sea, and from it billows 
of flame are tossed up to gigantic heights. These 
flaming jets are known under the name of pro- 
minences, because they were first noticed in the 
form of brilliant points projecting from behind the 
rim of the moon when the sun was totally eclipsed. 
Prominences are of two kinds, eruptive and quiescent. 
The eruptive prominences spurt up directly from the 
chromosphere with immense speeds, and change 
their shape with great rapidity. Quiescent promi- 
nences, on the other hand, have a form somewhat 
like trees, and alter their shape but slowly. In the 
eruptive prominences glowing masses of gas are 
shot up to altitudes sometimes as high as 300,000 
miles,^ with velocities even so great as from 500 
to 600 miles a second. It has been noticed that 
the eruptive prominences are mostly found in those 
portions of the sun where spots usually appear, 
namely, in the regions near the solar equator. The 
quiescent prominences, on the other hand, are con- 
fined, as a rule, to the neighbourhood of the sun's 
poles. 

1 On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at Oxford 
University, noted a prominence which rose to a height of 324,600 miles. 



Astronomy of To-day 

Prominences were at first never visible except during 
total eclipses of the sun. But in the year 1868, as 
we have already seen, a method of employing the 
spectroscope was devised, by means of which they 
could be observed and studied at any time, without 
the necessity of waiting for an eclipse. 

A still further development of the spectroscope, 
the Spectroheliograph, an instrument invented almost 
simultaneously by Professor Hale and the French 
astronomer, M. Deslandres, permits of photographs 
being taken of the sun, with the light emanating 
from only one of its glowing gases at a time. For 
instance, we can thus obtain a record of what the 
glowing hydrogen alone is doing, on the solar body 
at any particular moment. With this instrument it 
is also possible to obtain a series of photographs, 
showing what is taking place upon the sun at various 
levels. This is very useful in connection with the 
study of the spots ; for we are, in consequence, 
enabled to gather more evidence on the subject of 
their actual form than is given us by their highly 
foreshortened appearances when observed directly 
in the telescope. 

V. Corona. (Latin, a Crown.) 

This marvellous halo of pearly-white, light, which 
displays itself to our view only during the total 
phase of an eclipse of the sun, is by no means a 
layer like those other envelopments of the sun of 
which we have just been treating. It appears, on 
the other hand, to be composed of filmy matter, 
radiating outwards in every direction, and fading 

140 



The Sun 

away gradually into space. Its structure is noted 
to bear a strong resemblance to the tails of comets, 
or the streamers of the aurora borealis. 

Our knowledge concerning the corona has, how- 
ever, advanced very slowly. We have not, so far, 
been as fortunate with regard to it as with regard 
to the prominences ; and, for all we can gather 
concerning it, we are still entirely dependent upon 
the changes and chances of total solar eclipses. All 
attempts, in fact, to apply the spectroscopic method, 
so as to observe the corona at leisure in full sunlight 
in the way in which the prominences can be observed, 
have up to the present met with failure. 

The general form under which the corona appears 
to our eyes varies markedly at different eclipses. 
Sometimes its streamers are many, and radiate all 
round ; at other times they are confined only to the 
middle portions of the sun, and are very elongated, 
with short feathery-looking wisps adorning the solar 
poles. It is noticed that this change of shape varies 
in close accordance with that ii| year period during 
which the sun spots wax and wane ; the many- 
streamered regular type corresponding to the time 
of great sun-spot activity, while the irregular type 
with the long streamers is present only when the 
spots are few (see Plate VII., p. 142). Streamers 
have often been noted to issue from those regions 
of the sun where active prominences are at the 
moment in existence ; but it cannot be laid down 
that this is always the case. 

No hypothesis has yet been formulated which will 
account for the structure of the corona, or for its 
variation in shape. The great difficulty with regard 

141 



Astronomy of To-day 

to theorising upon this subject, is the fact that we 
see so much of the corona under conditions of 
marked foreshortening. Assuming, what indeed seems 
natural, that the rays of which it is composed issue 
in every direction from the solar body, in a manner 
which may be roughly imitated by sticking pins 
all over a ball ; it is plainly impossible to form any 
definite idea concerning .streamers, which actually 
may owe most of the shape they present to us, to 
the mixing up of multitudes of rays at all kinds of 
angles to the line of sight. In a word, w^e have* 
to try and form an opinion concerning an arrange- 
ment which, broadly speaking, is spherical^ but 
which, on account of its distance, must needs appear 
to us as absolutely yf<3:^. 

The most known about the composition of the 
corona is that it is made up of particles of matter, 
mingled with a glowing gas. It is an element in 
the composition of this gas which, as has been 
stated, is not found to tally with any known terrestrial 
element, and has, therefore, received the name of 
coronium for want of a better designation. 

One definite conclusion appears to be reached with 
regard to the corona, Le, that the matter of which 
it is composed, must be exceedingly rarefied ; as it 
is not found, for instance, to retard appreciably the 
speed of comets, on occasions when these bodies 
pass very close to the sun. A calculation has indeed 
been made which would tend to show that the 
particles composing the coronal matter, are separated 
from each other by a distance of perhaps between 
two and three yards 1 The density of the corona 
is found not to increase inwards towards the sun. 

142 




The Total Eclipse of the Sun of December 22nd, 1870 



Drawn by Mr. W. H. Wesley from a photograph taken at Syracuse by Mr. Brothers. 
This is the type of corona seen at the time oi greatest sunspot activity. The coronas of 
1882 (Plate I., p. 96) and of 1905 (Frontispiece) are of the same type. 




(B.) The Total Eclipse of the Sun of May 2Sth, 1900 

_ Drawn by Mr. W. H. Wesley from photographs taken by Mr. E. W. Maunder. This 
IS the type of corona seen when thesunspotsare least active. Compare the " Ring with 
Wings," Fig. 7, p. 87. 

Plate VII. Forms of the Solar Corona at the epochs of 
Sunspot INIaximum and Sunspot Minimum, respectively 

(.Page 141 



The Sun 

This is what has already been noted with regard to 
the layers lying beneath it. Powerful forces, acting 
in opposition to gravity, must hold sway here also. 

The II J year period, during which the sun spots 
vary in number and size, appears to govern the ac- 
tivities of the sun much in the same way that our 
year does the changing seasonal conditions of our 
earth. Not only, as we have seen, does the corona 
vary its shape in accordance with the said period, 
but the activity of the prominences, and of the faculae, 
follow suit. Further, this constant round of ebb and 
flow is not confined to the sun itself, but, strangely 
enough, affects the earth also. The displays of the 
aurora borealis, which we experience here, coincide 
closely with it, as does also the varying state of the 
earth's magnetism. The connection may be still 
better appreciated when a great spot, or group of 
spots, has made its appearance upon the sun. It 
has, for example, often been noted that when the 
solar rotation carries a spot, or group of spots, across 
the middle of the visible surface of the sun, our mag- 
netic and electrical arrangements are disturbed for 
the time being. The magnetic needles in our obser- 
vatories are, for instance, seen to oscillate violently, 
telegraphic communication is for a while upset, and 
magnificent displays of the aurora borealis illumine 
our night skies. Mr. E. W. Maunder, of Greenwich 
Observatory, who has made a very careful investiga- 
tion of this subject, suspects that, when elongated 
coronal streamers are whirled round in our direction 
by the solar rotation, powerful magnetic impulses may 
be projected upon us at the moments when such 
streamers are pointing towards the earth. 

143 



Astronomy of To-day 

Some interesting investigations with regard to sun- 
spots have recently been published by Mrs. E. W. 
Maunder. In an able paper, communicated to the 
Royal Astronomical Society on May lo, 1907; she 
reviews the Greenwich Observatory statistics dealing 
with the number and extent of the spots which have 
appeared during the period from 1889 to 1901 — a 
whole sun-spot cycle. From a detailed study of the 
dates in question, she finds that the number of those 
spots which are formed on the side of the sun turned 
away from us, and die out upon the side turned to- 
wards us, is much greater than the number of those 
which are formed on the side turned towards us and 
die out upon the side turned away. It used, for in- 
stance, to be considered that the influence of a planet 
might prodtcce sunspots ; but these investigations make 
it look rather as if some influence on the part of 
the earth tends, on the contrary, to extinguish them. 
Mrs. Maunder, so far, prefers to call the influence 
thus traced an apparent influence only, for, as she 
very fairly points out, it seems difficult to attribute a 
real influence in this matter to the earth, which is so 
small a thing in comparison not only with the sun, 
but even with many individual spots. 

The above investigation was to a certain degree 
anticipated by Mr. Henry Corder in 1895 ; but Mrs. 
Maunder's researches cover a much longer period, 
and the conclusions deduced are 'of a wider and more 
defined nature. 

With regard to its chemical composition, the spec- 
troscope shows us that thirty-nine of the elements 
which are found upon our earth are also to be found 
in the sun. Of these the best 'known are hydrogen, 

144 



The Sun 

oxygen^ helium, carbon, calcium, aluminium, iron, 
copper, zinc, silver, tin, and lead. Some elements of 
the metallic order have, however, not been found 
there, as, for instance, gold and mercury ; while a few 
of the other class of element, such as nitrogen, 
chlorine, and sulphur, are also absent. It must not, 
indeed, be concluded that the elements apparently 
missing do not exist at all in the solar body. Gold 
and mercury have, in consequence of their great 
atomic weight, perhaps sunk away into the centre. 
Again, the fact that we cannot find traces of certain 
other elements, is no real proof of their entire absence. 
Some of them may, for instance, be resolved into even 
simpler forms, under the unusual conditions which 
exist in the sun ; and so we are unable to trace them 
with the spectroscope, the experience of which rests 
on laboratory experiments conducted, at best, in con- 
ditions which obtain upon the earth. 



H5 



CHAPTER XIV 

THE INFERIOR PLANETS 

Starting from the centre of the solar system, the 
first body we meet with is the planet Mercury. It 
circulates at an average distance from the sun of 
about thirty-six millions of miles. The next body to 
it is the planet Venus, at about sixty-seven millions of 
miles, namely, about double the distance of Mercury 
from the sun. Since our earth comes next again, 
astronomers call those planets which circulate within 
its orbit, i.e. Mercury and Venus, the Inferior Planets, 
while those which circulate outside it they call the 
Superior Planets.^ 

In studying the inferior planets, the circumstances 
in which we make our observations are so very 
similar with regard to each, that it is best to take them 
together. Let us begin by considering the various 
positions of an inferior planet, as seen from the earth, 
during the course of its journeys round the sun. 
When furthest from us it is at the other side of the 
sun, and cannot then be seen owing to the blaze of 
light. As it continues its journey it passes to the left 
of the sun, and is then sufficiently away from the 
glare to be plainly seen. It next draws in again to- 

^ In employing the terms Inferior and Superior the writer bows to 
astronomical custom, though he cannot help feeling that, in the circum- 
stances, Interior and Exterior would be much more appropriate. 

146 



The Inferior Planets 

wards the sun, and is once more lost to view in the 
blaze at the time of its passing nearest to us. Then 
it gradually comes out to view on the right hand, 
separates from the sun up to a certain distance as 
before, and again recedes beyond the sun, and is for 
the time being once more lost to view. 

To these various positions technical names are 
given. When the inferior planet is on the far side 
of the sun from us, it is said to be in Superior Con- 
junction. When it has drawn as far as it can to the 
left hand, and is then as east as possible of the sun, 
it is said to be at its Greatest Eastern Elongation. 
Again, when it is passing nearest to us, it is said 
to be in Inferior Conjunction; and, finally, when it 
has drawn as far as it can to the right hand, it is 
spoken of as being at its Greatest Western Elongation 
(see Fig. ii, p. 148). 

The continual variation in the distance of an in- 
terior planet from us, during its revolution around the 
sun, will of course be productive of great alterations 
in its apparent size. At superior conjunction it ought, 
being then farthest away, to show the smallest disc ; 
while at inferior conjunction, being the nearest, it 
should look much larger. When at greatest elonga- 
tion, whether eastern or western, it should naturally 
present an appearance midway in size between 
the two. 

From the above considerations one would be in- 
clined to assume that the best time for studying the 
surface of an interior planet with the telescope is 
when it is at inferior conjunction, or, nearest to us. 
But that this is not the case will at once appear if we 
consider that the sunlight is then falling upon the side 

147 



Astronomy of To-day 

away from us, leaving the side which is towards us 
unillumined. In superior conjunction, on the other 
hand, the light falls full upon the side of the planet 

Various positions, and illumination by the Sun, of an Inferior Planet 
in the course of its orbit. 




0€i)#(13C)6 



Corresponding views of the same situations of an Inferior Planet as 
seen from the Earth, showing consequent phases and alterations 
in apparent size. 

Fig. II. — Orbit and Phases of an Inferior Planet. 

facing us ; but the disc is then so small-looking, and 
our view besides is so dazzled by the proximity of the 
sun, that observations are of little avail. In the elon- 
gations, however, the sunlight comes from the side, 

148 



The Inferior Planets 

and so we see one half of the^planet lit up ; the right 
half at eastern elongation, and the left half at western 
elongation. Piecing together the results given us at 
these more favourable views, we are enabled, bit by- 
bit, to gather some small knowledge concerning the 
surface of an inferior planet. 

From these considerations it will be seen at once 
that the inferior planets show various phases com- 
parable to the waxing and waning of our moon in 
its monthly round. Superior conjunction is, in fact, 
similar to full moon, and inferior conjunction to new 
moon ; while the eastern and western elongations may 
be compared respectively to the moon's first and last 
quarters. It will be recollected how, when these 
phases were first seen by the early telescopic ob- 
servers, the Copernican theory was felt to be im- 
mensely strengthened ; for it had been pointed out 
that if this system were the correct one, the planets 
Venus and Mercury, were it possible to see them 
more distinctly, would of necessity present phases like 
these when viewed from the earth. It should here be 
noted that the telescope was not invented until nearly 
seventy years after the death of Copernicus. 

The apparent swing of an inferior planet from side 
to side of the sun, at one time on the east side, then 
passing into and lost in the sun's rays to appear 
once more on the west side, is the explanation of 
what is meant when we speak of an evening or a 
morning star. An inferior planet is called an evening 
star when it is at its eastern elongation, that is to say, 
on the left-hand of the sun ; for, being then on the 
eastern side, it will set after the sun sets, as both sink 
in their turn below the western horizon at the close of 

149 



Astronomy of To-day 

day. Similarly, when such a planet is at its western 
elongation, that is to say, to the right-hand of the sun, 
it will go in advance of him, and so will rise above 
the eastern horizon before the sun rises, receiving 
therefore the designation of morning star. In very 
early times, Jiowever, before any definite ideas had 
been come to with regard to the celestial motions, it 
was generally believed that the morning and evening 
stars were quite distinct bodies. Thus Venus, when a 
morning star, w^as known to the ancients under the 
name of Phosphorus, or Lucifer ; v\^hereas they called 
it Hesperus when it was an evening star. 

Since an inferior planet circulates between us and 
the sun, one would be inclined to expect that such a 
body, each time it passed on the side nearest to the 
earth, should be seen as a black spot against the 
bright solar disc. Now this would most certainly be 
the case were the orbit of an inferior planet in the 
same plane with the orbit of the earth. But w^e have 
already seen how the orbits in the solar system, 
whether those of planets or of satellites, are by no 
means in the one plane ; and that it is for this very 
reason that the moon is able to pass time after time 
in the direction of the sun, at the epoch known as 
new moon, and yet not to eclipse him save after the 
lapse of several such passages. Transits, then, as the 
passages of an inferior planet across the sun's disc 
are called, take place, for the same reason, only after 
certain regular lapses of time ; and, as regards the 
circumstances of their occurrence, are on a par with 
eclipses of the sun. The latter, however, happen 
much more frequently, because the moon passes in 
the neighbourhood of the sun, roughly speaking, once 

150 



The Inferior Planets 

a month, whereas Venus conies to each inferior con- 
junction at intervals so long apart as a year and a 
half, and Mercury only about every four months. 
From this it will be further gathered that transits 
of Mercury take place much oftener than transits 
of Venus. 

Until recent years Transits of Venus wxre pheno- 
mena of great importance to astronomers, for they 
furnished the best means then available of calculat- 
ing the distance of the sun from the earth. This was 
arrived at through comparing the amount of apparent 
displacement in the planet's path across the solar 
disc, when the transit was observed from widely 
separated stations on the earth's surface. The last 
transit of Venus took place in 1882, and there will 
not be another until the year 2004. 

Transits of Mercury^ on the other hand, are not of 
much scientific importance. They are of no interest as 
a popular spectacle ; for the dimensions of the planet 
are so small, that it can be seen only with the aid 
of a telescope when it is in the act of crossing the 
sun's disc. The last transit of Mercury took place on 
November 14, 1907, and there will be another on 
November 6, -1914. 

The first person known to have observed a transit 
of an inferior planet was the celebrated French philo- 
sopher, Gassendi. This was the transit of Mercury 
which took place on the 7th of December 1631. 

The first time a transit of Venus was ever seen, so 
far as is known, was on the 24th of November 1639. 
The observer was a certain Jeremiah Horrox, curate 
of Hoole, near Preston, in Lancashire. The transit 
in question commenced shortly before sunset, and his 

151 



Astronomy of To-day 

observations in consequence were limited to only 
about half-an-hour. Horrox happened to have a 
great friend, one William Crabtree, of Manchester, 
whom he had advised by letter to be on the look 
out for the phenomenon. The weather in Crabtree's 
neighbourhood was cloudy, with the result that he 
only got a view of the transit for about ten minutes 
before the sun set. 

That this transit was observed at all is due entirely 
to the remarkable ability of Horrox. According to 
the calculations of the great Kepler, no transit could 
take place that year (1639), as the planet would just 
pass clear of the lower edge of the sun. Horrox, 
however, not being satisfied with this, worked the 
question out for himself, and came to the conclusion 
that the planet would actually traverse the lower 
portion of the sun's disc. The event, as we have 
seen, proved him to be quite in the right. Horrox 
is said to have been a veritable prodigy of astro- 
nomical skill ; and had he lived longer would, no 
doubt, have become very famous. Unfortunately he 
died about two years after his celebrated transit, in 
his twenty-second ytdiV only, according to the accounts. 
His friend Crabtree, who was then also a young man, 
is said to have been killed at the battle of Naseby 
in 1645. 

There is an interesting phenomenon in connection 
with transits which is known as the ''Black Drop." 
When an inferior planet has just made its way on to 
the face of the sun, it is usually seen to remain for 
a short time as if attached to the sun's edge by what 
looks like a dark ligament (see Fig. 12, p. 153). This 
gives to the planet for the time being an elongated 

152 



The Inferior Planets 

appearance, something like that of a pear ; but when 
the Hgament, which all the while keeps getting thinner 
and thinner, has at last broken, the black body of the 
planet is seen to stand outu'ound against the solar 
disc. 

This appearance may be roughly compared to the 
manner in which a drop of liquid (or, preferably, of 




Fig. 12.— The "Black Drop, 



some glutinous substance) tends for a while to adhere 
to an object from which it is falling. 

When the planet is in turn making its way off the 
face of the sun, the ligament is again seen to form 
and to attach it to the sun's edge before its due 
time. 

The phenomenon of the black drop, or ligament, is 
entirely an illusion, and, broadly speaking, of an optical 
origin. Something very similar will be noticed if one 

153 



Astronomy of To-day 

brings one's thumb and forefinger slowly together 
against a very bright background. 

This peculiar phenomenon has proved one of the 
greatest drawbacks to the proper observation of tran- 
sits, for it is quite impossible to note the exact instant 
of the planet's entrance upon and departure from the 
solar disc in conditions such as these. 

The black drop seems to bear a family relation, so 
to speak, to the phenomenon of Daily's beads. In 
the latter instance the lunar peaks, as they approach 
the sun's edge, appear to lengthen out in a similar 
manner and bridge the intervening space before their 
time, thus giving prominence to an effect which 
otherwise should scarcely be noticeable. 

The last transit of Mercury, which, as has been 
already stated, took place on November 14, 1907, was 
not successfully observed by astronomers in England, 
on account of the cloudiness of the weather. In 
France, however. Professor Moye, of Montpellier, saw 
it under good conditions, and mentions that the black 
drop remained very conspicuous for fully a minute. 
The transit was also observed in the United States, 
the reports from which speak of the black drop as 
very ^^troublesome." 

Before leaving the subject of transits it should be 
mentioned that it was in the capacity of commander 
of an expedition to Otaheite, in the Pacific, to observe 
the transit of Venus of June 3, 1769, that Captain 
Cook embarked upon the first of his celebrated 
voyages. 

In studying the surfaces of Venus and Mercury with 
the telescope, observers are, needless to say, very 
much hindered by the proximity of the sun. Venus, 

154 



The Inferior Planets 

when at the greatest elongations, certainly draws some 
distance out of the glare ; but her surface is, even 
then, so dazzlingly bright, that the markings upon it 
are difficult to see. Mercury, on the other hand, is 
much duller in contrast, but the disc it shows in the 
telescope is exceedingly small ; and, in addition, when 
that planet is left above the horizon for a short time 
after sunset, as necessarily happens after certain in- 
tervals, the mists near the earth's surface render 
observation of it very difficult. 

Until about twenty-five years ago, it was generally 
believed that both these planets rotated on their axes 
in about twenty-four hours, a notion, no doubt, 
originally founded upon an unconscious desire to 
bring them into some conformity with our earth. 
But Schiaparelli, observing in Italy, and Percival 
Lowell, in the clear skies of Arizona and Mexico, have 
lately come to the conclusion that both planets rotate 
upon their axes in the same time as they revolve in 
their orbits,^ the result being that they turn one 
face ever towards the sun in the same manner that 
the moon turns one face ever towards the earth — a 
curious state of things, which will be dealt with more 
fully when we come to treat of our satellite. 

The marked difference in the brightness between 
the two planets has already been alluded to. The 
surface of Venus is, indeed, about five times as bright 
as that of Mercury. The actual brightness of Mercury 
is about equivalent to that of our moon, and astro- 
nomers are, therefore, inclined to think that it may 

1 This question is, however, uncertain, for some very recent spectro- 
scopic observations of Venus seem to show a rotation period of about 
twenty-four hours. 



Astronomy of To-day 

resemble her in having a very rugged surface and 
practically no atmosphere. This probable lack of 
atmosphere is further corroborated by two circum- 
stances. One of these is that when Mercury is just 
about to transit the face of the sun, no ring of 
diffused light is seen to encircle its disc as would 
be the case if it possessed an atmosphere. Such a 
lack of atmosphere is, indeed, only to be expected 
from what is known as the Kinetic Theory of Gases, 
According to this theory, which is based upon the 
behaviour of various kinds of gas, it is found that 
these elements tend to escape into space from the 
surface of bodies whose force of gravitation is weak. 
Hydrogen gas, for example, tends to fly away from 
our earth, as any one may see for himself when a 
balloon rises into the air. The gravitation of the 
earth seems, however, powerful enough to hold down 
other gases, as, for instance, those of which the air is 
chiefly composed, namely, oxygen and nitrogen. In 
due accordance with the Kinetic theory, we find the 
moon and Mercury, which are much about the same 
size, destitute of atmospheres. Mars, too, .whose 
diameter is only about double that of the moon, 
has very little atmosphere. We find, on the other 
hand, that Venus, which is about the same size as our 
earth, clearly possesses an atmosphere, as just before 
the planet is in transit across the sun, the outline of 
its dark body is seen to be surrounded by a bright 
ring of light. 

The results of telescopic observation show that 
more markings are visible on Mercury than on Venus. 
The intense brilliancy of Venus is, indeed, about the 
same as that of our white clouds when the sun is 

156 



The Inferior Planets 

shining directly upon them. It has, therefore, been 
supposed that the planet is thickly enveloped in 
cloud, and that we do not ever see any part of its 
surface, except perchance the summit of some lofty 
mountain projecting through the fleecy mass. 

With regard to the great brilliancy of Venus, it may 
be mentioned that she has frequently been seen in 
England, with the naked eye in full sunshine, when 
at the time of her greatest brightness. The writer 
has seen her thus at noonday. Needless to say, the 
sky at the moment was intensely blue and clear. 

The orbit of Mercury is very oval, and much more 
so than that of any other planet. The consequence is 
that, when Mercury is nearest to the sun, the heat 
which it receives is twice as great as when it is 
farthest away. The orbit of Venus, on the other 
hand, is in marked contrast with that of Mercury, 
and is, besides, more nearly of a circular shape 
than that of any of the other planets. Venus, there- 
fore, always keeps about the same distance from the 
sun, and so the heat which she receives during the 
course of her year can only be subject to very slight 
variations. 



157 



CHAPTER XV 

THE EARTH 

We have already seen (in Chapter I.) how, in very 
early times, men naturally enough considered the 
earth to be a flat plane extending to a very great 
distance in every direction ; but that, as years went 
on, certain of the Greek philosophers suspected it 
to be a sphere. One or two. of the latter are, indeed, 
said to have further believed in its rotation about an 
axis, and even in its revolution around the sun ; but, 
as the ideas in question were founded upon fancy, 
rather than upon any direct evidence, they did not 
generally attract attention. The small effect, there- 
fore, which these theories had upon astronomy, may 
well be gathered from the fact that in the Ptolemaic 
system the earth was considered as fixed and at the 
centre of things; and this belief, as we have seen, 
continued unaltered down to the days of Copernicus. 
It was, indeed, quite impossible to be certain of the 
real shape of the earth or the reality of its motions 
until knowledge became more extended and scientific 
instruments much greater in precision. 

We will now consider in detail a few of the more 
obvious arguments which can be put forward to show 
that our earth is a sphere. 

If, for instance, the earth were a plane surface, a 
ship sailing away from us over the sea would appear 

158 



The Earth 

to grow smaller and smaller as it receded into the 
distance, becoming eventually a tiny speck, and 
fading gradually from our view. This, however, is 
not at all what actually takes place. As we watch a 
vessel receding, its hull appears bit by bit to slip 
gently down over the horizon, leaving the masts alone 
visible. Then, in their turn, the masts are seen to 
slip down in the same manner, until eventually every 
trace of the vessel is gone. On the other hand, when 
a ship comes into view, the masts are the first portions 
to appear. They gradually rise up from below the 
horizon, and the hull follows in its turn, until the 
whole vessel is visible. Again, when one is upon a 
ship at sea, a set of masts will often be seen sticking 
up alone above the horizon, and these may shorten 
and gradually disappear from view without the body 
of the ship to which they belong becoming visible at 
all. Since one knows from experience that there is 
no edge at the horizon over which a vessel can drop 
down, the appearance which we have been describing 
can only be explained by supposing that the surface 
of the earth is always curving gradually in every 
direction. 

The distance at which what is known as the horizon 
lies away from us depends entirely upon the height 
above the earth's surface where we happen at the 
moment to be. A ship which has appeared to sink 
below the horizon for a person standing on the beach, 
will be found to come back again into view if he at 
once ascends a high hill. Experiment shows that 
the horizon line lies at about three miles away for a 
person standing at the water's edge. The curving of 
the earth's surface is found, indeed, to be at the rate 

^59 



Astronomy of To-day 

of eight inches in every mile. Now it can be ascer- 
tained, by calculation, that a body curving at this rate 
in every direction must be a globe about 8000 miles 
in diameter. 

Again, the fact that, if not stopped by such insuper- 
able obstacles as the polar ice and snow, those who 
travel continually in any one direction upon the earth's 
surface always find themselves back again at the 
regions from which ithey originally set out, is additional 
ground for concluding that the earth is a globe. 

We can find still further evidence. For instance, 
in an eclipse of the moon the earth's shadow, when 
seen creeping across the moon's face, is noted to be 
always circular in shape. One cannot imagine how 
such a thing could take place unless the earth were a 
sphere. 

Also, it is found from observation that the sun, the 
planets, and the satellites are, all of them, round. 
This roundness cannot be the roundness of a flat 
plate, for instance, for then the objects in question 
would sometimes present their thin sides to our view. 
It happens, also, that upon the discs which these 
bodies show, we see certain markings shifting along 
continually in one direction, to disappear at one side 
and to reappear again at the other. Such bodies 
must, indeed, be spheres in rotation. 

The crescent and other phases, shown by the moon 
and the inferior planets, should further impress the 
truth of the matter upon us, as such appearances can 
only be caused by the sunlight falling from various 
directions upon the surfaces of spherical bodies. 

Another proof, perhaps indeed the weightiest of 
all, is the continuous manner in which the stars over- 

160 



The Earth 

head give place to others as one travels about the 
surface of the earth. When in northern regions the 
Pole Star and its neighbours — the stars composing 
the Plough, for instance — are over our heads. As 
one journeys south these gradually sink towards the 
northern horizon, while other stars take their place, 
and yet others are uncovered to view from the south. 
The regularity with which these changes occur shows 
that every point on the earth's surface faces a different 
direction of the sky, and such an arrangement would 
only be possible if the earth were a sphere. The 
celebrated Greek philosopher, Aristotle, is known to 
have believed in the globular shape of the earth, and 
it was by this very argument that he had convinced 
himself that it was so. 

The idea of the sphericity of the earth does not 
appear, hawever, to have been generally accepted 
until the voyages of the great navigators showed that 
it could be sailed round. 

The next point we have to consider is the rota- 
tion of the earth about its axis. From the earliest 
times men noticed that the sky and everything in 
it appeared to revolve around the earth in one fixed 
direction, namely, towards what is called the West, 
and that it made one complete revolution in the 
period of time which we know as twenty-four hours. 
The stars were seen to come up, one after another, 
from below ^-he eastern horizon, to mount the sky, and 
then to sink in turn below the western horizon. The 
sun was seen to perform exactly the same journey, 
and the moon, too, whenever she was visible. One 
or two of the ancient Greek philosophers perceived 
that this might be explained, either by a movement 

i6i L 



Astronomy of To-day 

of the entire heavens around the earth, or by a 
turning motion on the part of the earth itself. Of 
these diverse explanations, that which supposed an 
actual movement of the heavens appealed to them 
the most, for they could hardly conceive that the 
earth should continually rotate and men not be 
aware of its movement. The question may be com- 
pared to what we experience when borne along in a 
railway train. We see the telegraph posts and the 
trees and buildings near the line fly past us one after 
another in the contrary direction. Either these must 
be moving, or we must be moving ; and as w^e 
happen to kjiow that it is, indeed, we who are moving, 
there can be no question therefore about the matter. 
But it would not be at all so easy to be sure of this 
movement were one unable to see the objects close 
at hand displacing themselves. For instance, if one 
is shut up in a railway carriage at night with the 
blinds down, there is really nothing to show that 
one is moving, except the jolting of the train. And 
even then it is hard to be sure in w^hich direction one 
is actually travelling. 

The way w^e are situated upon the earth is therefore 
as follows. There are no other bodies sufficiently 
near to be seen flying past us in turn ; our earth 
spins without a jolt ; we and all things around us, 
including the atmosphere itself, are borne along to- 
gether with precisely the same impetus, just as all 
the objects scattered about a railway carriage share 
in the forward movement of the train. Such being 
the case, what wonder that we are unconscious of 
the earth's rotation, of which we should know 
nothing at all, were it not for that slow displacement 

162 



The Earth 

of the distant objects in the heavens, as we are borne 
past them in turn. 

If the night sky be watched, it will be soon found 
that its apparent turning movement seems to take 
place around a certain point, which appears as if 
fixed. This point is known as the north pole of the 
heavens ; and a rather bright star, which is situated 
very close to this hub of movement, is in consequence 
called the Pole Star. For the dwellers in southern 
latitudes there is also a point in their sky which 
appears to remain similarly fixed, and this is known 
as the south pole of the heavens. Since, however, the 
heavens do not turn round at all, but the earth does, 
it will easily be seen that these apparently stationary 
regions in the sky are really the points towards 
which the axis of the earth is directed. The positions 
on the earth's surface itself, known as the North and 
South Poles, are merely the places where the earth's 
axis, if there were actually such a thing, would be 
expected to jut out. The north pole of the earth will 
thus be situated exactly beneath the north pole of the 
heavens, and the south pole of the earth exactly 
beneath the south pole of the heavens. 

We have seen that the earth rotates upon its 
imaginary axis once in about every twenty-four hours. 
This means that everything upon the surface of the 
earth is carried round once during that time. The 
measurement around the earth's equator is about 
24,000 miles ; and, therefore, an object situated at the 
equator must be carried round through a distance of 
about 24,000 miles in each twenty-four hours. Every- 
thing at the equator is thus moving along at the rapid 
rate of about 1000 miles an hour, or between sixteen 

163 



Astronomy of To-day 

and seventeen times as fast as an express train. If, 
however, one were to take measurements around the 
earth parallel to the equator, one would find these 
measurements becoming less and less, according as 
the poles were approached. It is plain, therefore, 
that the speed with which any point moves, in conse- 
quence of the earth's rotation, will be greatest at the 
equator, and less and less in the direction of the 
poles ; while at the poles themselves there will be 
practically no movement, and objects there situated 
will merely turn round. 

The considerations above set forth, with regard to 
the different speeds at which different portions of a 
rotating globe will necessarily be moving, is the 
foundation of an interesting experiment, which gives 
us further evidence of the rotation of our earth. The 
measurement around the earth at any distance below 
the surface, say, for instance, at the depth of a mile, 
will clearly be less than a similar measurement at the 
surface itself. The speed of a point at the bottom of 
a mine, which results from the actual rotation of the 
earth, must therefore be less than the speed of a 
point at the surface overhead. This can be definitely 
proved by dropping a heavy object down a mine 
shaft. The object, which starts with the greater 
speed of the surface, will, when it reaches the bottom 
of the mine, be found, as might be indeed expected, to 
be a little ahead {i,e. to the east) of the point which 
originally lay exactly underneath it. The distance 
by which the object gains upon this point is, how- 
ever, very small. In our latitudes it amounts to 
about an inch in a fall of 500 feet. 

The great speed at which, as we have seen, the 

164 



The Earth 

equatorial regions of the earth are moving, should 
result in giving to the matter there situated a certain 
tendency to fly outwards. Sir Isaac Newton was the 
first to appreciate this point, and he concluded from 
it that the earth must be bulged a little all round the 
equator. This is, indeed, found to be the case, the 
diameter at the equator being nearly twenty-seven 
miles greater than it is from pole to pole. The reader 
will, no doubt, be here reminded of the familiar com- 
parison in geographies between the shape of the earth 
and that of an orange. 

In this connection it is interesting to consider that, 
w^ere the earth to rotate seventeen times as fast as 
it does (i.e. in one hour twenty-five minutes, instead 
of twenty-four hours), bodies at the equator would 
have such a strong tendency to fly outwards that the 
force of terrestrial gravity acting upon them would 
just be counterpoised, and they would virtually have 
no zueight. And, further, were the earth to rotate a 
little faster still, objects lying loose upon its surface 
would be shot off into space. 

The earth is, therefore, what is technically known 
as an oblate spheroid ; that is, a body of spherical shape 
flattened at the poles. It follows of course from this, 
that objects at the polar regions are slightly nearer 
to the earth's centre than objects at the equatorial 
regions. We have already seen that gravitation acts 
from the central parts of a body, and that its force is 
greater the nearer are those central parts. The 
result of this upon our earth will plainly be that 
objects in the polar regions will be pulled with a 
slightly stronger pull, and will therefore weigh a trifle 
more than objects in the equatorial regions. This is, 

165 



Astronomy of To-day 

indeed, found by actual experiment to be the case. 
As an example of the difference in question, Professor 
Young, in his Manual of Astronomy y points out that 
a man who weighs 190 pounds at the equator would 
weigh 191 at the pole. In such an experiment the 
weighing would, however, have to be made with a 
spring balance^ and not with scales ; for, in the latter 
case, the ^^ weights " used would alter in their weight 
in exactly the same degree as the objects to be 
weighed. 

It used to be thought that the earth was composed 
of a relatively thin crust, with a molten interior. 
Scientific men now believe, on the other hand, that 
such a condition cannot after all prevail, and that 
the earth must be more or less solid all through, 
except perhaps in certain isolated places where col- 
lections of molten matter may exist. 

The atmosphere, or air which we breathe, is in the 
form of a layer of limited depth which closely en- 
velops the earth. Actually, it is a mixture of several 
gases, the most important being nitrogen and oxygen, 
which between them practically make up the air, 
for the proportion of the other gases, the chief of 
which is carbonic acid gas, is exceedingly small. 

It is hard to picture our earth, as we know it, 
without this atmosphere. Deprived of it, men at once 
would die ; but even if they could be made to go on 
living without it by any miraculous means, they would 
be like unto deaf beings, for they would never hear 
any sound. What we call sounds are merely vibrations 
set up in the air, which travel along and strike upon 
the drum of the ear. 

The atmosphere is densest near the surface of the 

166 



The Earth 

earth, and becomes less and less dense away from it, 
as a result of diminishing pressure of air from above. 
The greater portion of it is accumulated within four 
or five miles of the earth's surface. 

It is impossible to determine exactly at what distance 
from the earth's surface the air ceases altogether, 
for it grows continually more and more rarefied. 
There are, however, two distinct methods of ascertain- 
ing the distance beyond which it can be said practi- 
cally not to exist. One of these methods we get from 
twilight. Twilight is, in fact, merely light reflected 
to us from those upper regions of the air, which still 
continue to be illuminated by the sun after it has 
disappeared from our view below the horizon. The 
time during which twilight lasts, shows us that the 
atmosphere must be at least fifty miles high. 

But the most satisfactory method of ascertaining 
the height to which the atmosphere extends is from 
the observation of meteors. It is found that these 
bodies become ignited, by the friction of passing into 
the atmosphere, at a height of about loo miles above 
the surface of the earth. We thus gather that the 
atmosphere has a certain degree of density even at 
this height. It may, indeed, extend as far as about 
150 miles. 

The layer of atmosphere surrounding our earth acts 
somewhat in the manner of the glass covering of a 
greenhouse, bottling in the sun's rays, and thus 
storing up their warmth for our benefit. Were this 
not so, the heat which we get from the sun would, 
after falling upon the earth, be quickly radiated again 
into space. 

It is owing to the unsteadiness of the air that stars 

167 



Astronomy of To-day 

are seen to twinkle. A night when this takes place, 
though it may please the average person, is worse 
than useless to the astronomer, for the unsteadi- 
ness is greatly magnified in the telescope. This 
twinkling is, no doubt, in a great measure responsible 
for the conventional '' points " with which Art has 
elected to embellish stars, and which, of course, have 
no existence in fact. 

The phenomena of Refraction, ^ namely, that bending 
which rays of light undergo, when passing slantwise 
from a rare into a dense transparent medium, are 
very marked with regard to the atmosphere. The 
denser the medium into which such rays pass, the 
greater is this bending found to be. Since the layer 
of air around us becomes denser and denser towards 
the surface of the earth, it will readily be granted 
that the rays of light reaching our eyes from a celestial 
object, will suffer the greater bending the lower the 
object happens to be in the sky. Celestial objects, 
unless situated directly overhead, are thus not seen 
in their true places, and when nearest to the horizon 
are most out of place. The bending alluded to is 
upwards. Thus the sun and the moon, for instance, 
when we see them resting upon the horizon, are 
actually entirely beneath it. 

When the sun, too, is sinking towards the horizon, 
the lower edge of its disc will, for the above reason, 

^ Every one knows the simple experiment in which a coin lying at the 
bottom of an empty basin, and hidden from the eye by its side, becomes 
visible when a certain quantity of water has been poured in. This is an 
example of refraction. The rays of light coming from the coin ought not 
to reach the eye, on account of the basin's side being in the way ; yet by 
the action of the water they are refracted^ or bent over its edge, in such 
a manner that they do. 

i68 



The Earth 

look somewhat more raised than the upper. The 
result is a certain appearance of flattening ; which 
may plainly be seen by any one who watches the orb 
at setting. 

In observations to determine the exact positions of 
celestial objects correction has to be made for the 
effects of refraction, according to the apparent eleva- 
tion of these objects in the sky. Such effects are 
least when the objects in question are directly over- 
head, for then the rays of light, coming from them to 
the eye, enter the atmosphere perpendicularly, and 
not at any slant. 

A very curious effect, due to refraction, has occa- 
sionally been observed during a total eclipse of the 
moon. To produce an eclipse of this kind, the earth 
must, of course^ lie directly between the sun and the moon. 
Therefore, when we see the shadow creeping over the 
moon's surface, the sun should actually be well below 
the horizon. But when a lunar eclipse happens to 
come on just about sunset, the sun, although really 
sunk below the horizon, appears still above it through 
refraction, and the eclipsed moon, situated, of course, 
exactly opposite to it in the sky, is also lifted up above 
the horizon by the same cause. Pliny, writing in the 
first century of the Christian era, describes an eclipse 
of this kind, and refers to it as a ^'prodigy." The 
phenomenon is known as a "horizontal eclipse." 
It was, no doubt, partly owing to it that the ancients 
took so long to decide that an eclipse of the moon 
was really caused by the shadow cast by the earth. 
Plutarch, indeed, remarks that it was easy enough to 
understand that a solar eclipse was caused by the 
interposition of the moon, but that one could not 

169 



Astronomy of To-day 

imagine by the interposition of what body the moon 
itself could be eclipsed. 

In that apparent movement of the heavens about 
the earth, which men now know to be caused by the 
mere rotation of the earth itself, a slight change is 
observed to be continually taking place. The stars, 
indeed, are always found to be gradually drawing 
westward, i,e. towards the sun, and losing themselves 
one after the other in the blaze of his light, only to 
reappear, however, on the other side of him after a 
certain lapse of time. This is equivalent to saying 
that the sun itself seems always creeping slowly 
eastward in the heaven. The rate at which this 
appears to take place is such that the sun finds 
itself back again to its original position, with regard to 
the starry background, at the end of a year's time. In 
other words, the sun seems to make a complete tour 
of the heavens in the course of a year. Here, how- 
ever, we have another illusion, just as the daily move- 
ment of the sky around the earth was an illusion. 
The truth indeed is, that this apparent movement 
of the sun eastward among the stars during a year, 
arises merely from a continuous displacement of his 
position caused by an actual motion of the earth itself 
around him in that very time. In a word, it is the 
earth which really moves around the sun, and not 
the sun around the earth. 

The stress laid upon this fundamental point by 
Copernicus, marks the separation of the modern 
from the ancient view. Not that Copernicus, indeed, 
had obtained any real proof that the earth is merely 
a planet revolving around the sun ; but it seemed to 
his profound intellect that a movement of this kind 

170 



The Earth 

on the part of oar globe was the more Hkely explana- 
tion of the celestial riddle. The idea was not new ; 
for, as we have already seen, certain of the ancient 
Greeks (Aristarchus of Samos, for example) had held 
such a view ; but their notions on the subject were 
very fanciful, and unsupported by any good argument. 

What Copernicus, however, really seems to have 
done was to insist upon the idea that the sun occu- 
pied the centre^ as being more consonant with common 
sense. No doubt, he was led to take up this position 
by the fact that the sun appeared entirely of a different 
character from, the other members of the system. 
The one body in the scheme, which performed the 
important function of dispenser of light and heat, 
would indeed be more likely to occupy a position 
apart from the rest ; and what position more appro- 
priate for its purposes than the centre ! 

But here Copernicus only partially solved the diffi- 
cult question. He unfortunately still clung to an 
ancient belief, which as yet remained unquestioned ; 
i.e, the great virtue, one might almost say, the divine- 
nesSf of circular motion. The ancients had been hag- 
ridden, so to speak, by the circle ; and it appeared 
to them that such a perfectly formed curve was alone 
fitted for the celestial motions. Ptolemy employed 
it throughout his system. According to him the 
^' planets " (which included, under the ancient view, 
both the sun and the moon), moved around the earth 
in circles ; but, as their changing positions in the sky 
could not be altogether accounted for in this way, 
it was further supposed that they performed addi- 
tional circular movements, around peculiarly placed 
centres, during the course of their orbital revolutions. 

171 



Astronomy of To-day 

Thus the Ptolemaic system grew to be extremely 
complicated ; for astronomers did not hesitate to 
add new circular movements whenever the celestial 
positions calculated for the planets w^ere found not 
to tally with the positions observed. In this manner, 
indeed, they succeeded in doctoring the theory, so 
that it fairly satisfied the observations made w4th the 
rough instruments of pre-telescopic times. 

Although Copernicus performed the immense ser- 
vice to astronomy of boldly directing general attention 
to the central position of the sun, he unfortunately 
took over for the new scheme the circular machinery 
of the Ptolemaic system. It therefore remained for 
the famous Kepler, who lived about a century after 
him, to find the complete solution. Just as Coper- 
nicus, for instance, had broken free from tradition 
with regard to the place of the sun ; so did Kepler, 
in turn, break free from the spell of circular motion, 
and thus set the coping-stone to the new astronomical 
edifice. This astronomer showed, in fact, that if 
the paths of the planets around the sun, and of the 
moon around the earth, were not circles, but ellipsesy 
the movements of these bodies about the sky could 
be correctly accounted for. The extreme simplicity 
of such an arrangement was far more acceptable 
than the bewildering intricacy of movement required 
by the Ptolemaic theory. The Copernican system, 
as amended by Kepler, therefore carried the day ; 
and was further strengthened, as we have already 
seen, by the telescopic observations of Galileo and 
the researches of Newton into the effects of gravi- 
tation. 

And here a word on the circle, now fallen from 
172 



The Earth 

its high estate. The ancients were in error in sup- 
posing that it stood entirely apart — the curve of 
curves. As a matter of fact it is merely a special kind 
of ellipse. To put it paradoxically^ it is an ellipse 
which has no ellipticity, an oval without any ovalness ! 
Notwithstanding all this, astronomy had to wait 
yet a long time for a definite proof of the revolution 
of the earth around the sun. The leading argument 
advanced by Aristotle, against the reality of any 
movement of the earth, still held good up to about 
seventy years ago. That philosopher had pointed 
out that the earth could not move about in space 
to any great extent, or the stars would be found to 
alter their apparent places in the sky, a thing which 
had never been observed to happen. Centuries ran 
on, and instruments became more and more perfect, 
yet no displacements of stars were noted. In accept- 
ing the Copernican theory men were therefore obliged 
to suppose these objects as immeasurably distant. 
At length, however, between the years 1835 and 1840, 
it was discovered by the Prussian astronomer, Bessel, 
that a star known as 61 Cygni — that is to say, the 
star marked in celestial atlases as No. 61 in the 
constellation of the Swan — appeared, during the 
course of a year, to perform a tiny circle in the 
heavens, such as would result from a movement on 
our own part around the sun. Since then about 
forty-three stars have been found to show minute 
displacements of a similar kind, which cannot be 
accounted for upon any other supposition than that 
of a continuous revolution of the earth around the 
sun. The triumph of the Copernican system is now 
at last supreme. 

173 



Astronomy of To-day 

If the axis of the earth stood ^' straight up/' so to 
speak, while the earth revolved in its orbit, the sun 
would plainly keep always on a level with the equator. 
This is equivalent to stating that, in such circum- 
stances, a person at the equator would see it rise 
each morning exactly in the east, pass through the 
zenith^ that is, the point directly overhead of him, at 
midday, and set in the evening due in the west. As 
this would go on unchangingly at the equator every 
day throughout the year, it should be clear that, 
at any particular place upon the earth, the sun would 
in these conditions always be seen to move in an 
unvarying manner across the sky at a certain alti- 
tude depending upon the latitude of the place. Thus 
the more north one went upon the earth's surface, 
the more southerly in the sky would the sun's path 
lie ; while at the north pole itself, the sun would 
always run round and round the horizon. Similarly, 
the more south one went from the equator the more 
northerly would the path of the sun lie, while at the 
south pole it would be seen to skirt the horizon in 
the same manner as at the north pole. The result 
of such an arrangement would be, that each place 
upon the earth would always have one unvarying 
climate ; in which case there would not exist any 
of those beneficial changes of season to which we 
owe so much. 

The changes of season, which we fortunately ex- 
perience, are due, however, to the fact that the sun 
does not appear to move across the sky each day 
at one unvarying altitude, but is continually altering 
the position of its path ; so that at one period of the 
year it passes across the sky low down, and remains 

174 



The Earth 

above the horizon for a short time only, while at 
another it moves high up across the heavens, and is 
above the horizon for a much longer time. Actually, 
the sun seems little by little to creep up the sky 
during one half of the year, namely, from mid-winter 
to mid-summer, and then, just as gradually, to slip 
down it again during the other half, namely, from 
mid-summer to mid-winter. It will therefore be clear 
that every region of the earth is much more thoroughly 
warmed during one portion of the year than during 
another, i,e, when the sun's path is high in the heavens 
than when it is low down. 

Once more we find appearances exactly the con- 
trary from the truth. The earth is in this case the 
real cause of the deception, just as it was in the other 
cases. The sun does not actually creep slowly up 
the sky, and then slowly dip down it again, but, 
owing to the earth's axis being set aslant, different 
regions of the earth's surface are presented to the 
sun at different times. Thus, in one portion of its 
orbit, the northerly regions of the earth are presented 
to the sun, and in the other portion the southerly. 
It follows of course from this, that when it is summer 
in the northern hemisphere it is winter in the southern, 
and vice versd (see Fig. 13, p. 176). 

The fact that, in consequence of this slant of the 
earth's axis, the sun is for part of the year on the 
north side of the equator and part of the year on 
the south side, leads to a very peculiar result. The 
path of the moon around the earth is nearly on the 
same plane with the earth's path around the sun. 
The moon, therefore, always keeps to the same regions 
of the sky as the sun. The slant of the earth's axis 

175 



Astronomy of To-day 

thus regularly displaces the position of both the sun 
and the moon to the north and south sides of the 
equator respectively in the manner we have been de- 
scribing. Were the earth, however, a perfect sphere, 
such change of position would not produce any effect. 
We have shown, however, that the earth is not a 
perfect sphere, but that it is bulged out all round the 
equator. The result is that this bulged-out portion 
swings slowly under the pulls of solar and lunar 

Orbit ofEarf/i 



ijune. 
Af fiale/i/^senled ti'fi/vrds 
Sun and S. Me turned , ' 
aimi/ /rem it. , ' 



Xly/J.fhh 




S.PotS. 



Summer there fore in ~ , 
the .Northern Nemts^/nA- ■ , 
cmdWinterin the boutA^n ' 




December. 
S Po'e pi'esented tcmrt/i 
Sun andiVJhte turned 
- ^ a nay from it. 



S.Po. 




, ' Summer thure/bre in 
^ ' the Soulher/i Memisp/uirs 
and Wintayin the A/urthem, 



Orfaitof F.orlh 

Fig. 13. — Summer and Winter. 



gravitation, in response to the displacements of the 
sun and moon to the north and to the south of it. 
This slow swing of the equatorial regions results, of 
course, in a certain slow change of the direction 
of the earth's axis, so that the north pole does not 
go on pointing continually to the same region of the 
sky. The change in the direction of the axis is, 
however, so extremely slight, that it shows up only 
after the lapse of ages. The north pole of the 
heavens, that is, the region of the sky towards which 
the north pole of the earth's axis points, displaces 

176 



The Earth 

therefore extremely slowly, tracing out a wide circle, 
and arriving back again to the same position in the 
sky only after a period of about 25,000 years. At 
present the north pole of the heavens is quite close 
to a bright star in the tail of the constellation of 
the Little Bear, which is consequently known as the 
Pole Star ; but in early Greek times it was at least 
ten times as far away from this star as it is now. 
After some 12,000 years the pole will point to the 
constellation of Lyra, and Vega, the most brilliant 
star in that constellation, '^ill then be considered as 
the pole star. This slow twisting of the earth's axis 
is technically known as Precession, or the Precession 
of the Equinoxes (see Plate XIX., p. 292). 

The slow displacement of the celestial pole appears 
to have attracted the attention of men in very early 
times, but it was not until the second century B.C. 
that precession was established as a fact by the cele- 
brated Greek astronomer, Hipparchus. For the 
ancients this strange cyclical movement had a mystic 
significance ; and they looked towards the end of the 
period as the end, so to speak, of a ^' dispensation," 
after which the life of the universe would begin 
anew : — 

" Magnus ab integro s£eclorum nascitur ordo. 
Jam redit et Virgo, redeunt Saturnia regna ; 

Alter erit turn Tiphys, et altera quae vehat Argo 

Delectos heroas ; erunt etiam altera bella, 

Atque iterum ad Trojam magnus mittetur Achilles." 

We have seen that the orbit of the earth is an 
ellipse, and that the sun is situated at what is called 
the focusy a point not in the middle of the ellipse, 

177 M 



Astronomy of To-day 

but rather towards one of its ends. Therefore, during 
the course of the year the distance of the earth from 
the sun varies. The sun, in consequence of this, is 
about 3,000,000 miles nearer to us in our northern 
winter than it is in our northern summer, a statement 
which sounds somewhat paradoxical. This variation 
in distance, large as it appears in figures, can, how- 
ever, not be productive of much alteration in the 
amount of solar heat which we receive,- for during 
the first week in January, when the distance is least, 
the sun only looks about one-eighteenth broader than 
at the commencement of July, when the distance is 
greatest. The great disparity in temperature between 
winter and summer depends, as we have seen, upon 
causes of quite another kind, and varies between 
such wide limits that the effects of this slight altera- 
tion in the distance of the sun from the earth may 
be neglected for practical purposes. 

The Tides are caused by the gravitational pull of 
the sun and moon upon the water of the earth's 
surface. Of the two, the moon, being so much the 
nearer, exerts the stronger pull, and therefore may 
be regarded as the chief cause of the tides. This 
pull always draws that portion of the water, which 
happens to be right underneath the moon at the time, 
into a heap ; and there is also a second heaping of 
water at the same moment at the contrary side of the 
earth, the reasons for which can be shown mathe- 
matically, but cannot be conveniently dealt with 
here. 

As the earth rotates on its axis each portion of its 
surface passes beneath the moon, and is swelled up 
by this pull ; the watery portions being, however, the 

178 



The Earth 

only ones to yield visibly. A similar swelling up, as 
we have seen, takes place at the point exactly away 
from the moon. Thus each portion of our globe is 
borne by the rotation through two "tide-areas" every 
day, and this is the reason why there are two tides 
during every twenty-four hours. 

The crest of the watery swelling is known as high 
tide. The journey of the moon around the earth takes 
about a month, and this brings her past each place 
in turn by about fifty minutes later each day, which 
is the reason why high tide is usually about twenty- 
five minutes later each time. 

The moon is, however, not the sole cause of the 
tides, but the sun, as we have said, has a part in the 
matter also. When it is new moon the gravitational 
attractions of both sun and moon are clearly acting 
together from precisely the same direction, and, there- 
fore, the tide will be pulled up higher than at other 
times. At full moon, too, the same thing happens ; 
for, although the bodies are now acting from opposite 
directions, they do not neutralise each other's pulls 
as one might imagine, since the sun, in the same 
manner as the moon, produces a tide both under it 
and also at the opposite side of the earth. Thus both 
these tides are actually increased in height. The ex- 
ceptionally high tides which we experience at new 
and full moons are known as Spring TideSf in contra- 
distinction to the minimum high tides, which are 
known as Neap Tides. 

The ancients appear to have had some idea of the 
cause of the tides. It is said that as early as looo B.C. 
the Chinese noticed that the moon exerted an influ- 
ence upon the waters of the sea. The Greeks and 

179 



Astronomy of To-day 

Romans, too, had noticed the same thing ; and Caesar 
tells us that when he was embarking his troops for 
Britain the tide was high because the moon was full. 
Pliny went even further than this, in recognising a 
similar connection between the waters and the sun. 

From casual observation one is inclined to suppose 
that the high tide always rises many feet. But that 
this is not the case is evidenced by the fact that the 
tides in the midst of the great oceans are only from 
three to four feet high. However, in the seas and 
straits around our Isles, for instance, the tides rise 
very many feet indeed, but this is merely owing to the 
extra heaping up which the large volumes of water 
undergo in forcing their passage through narrow 
channels. 

As the earth, in rotating, is continually passing 
through these tide-areas, one might expect that the 
friction thus set up would tend to slow down the 
rotation itself. Such a slowing down, or '4idd drag," 
as it is called, is indeed continually going on ; but the 
effects produced are so exceedingly minute that it will 
take many millions of years to make the rotation 
appreciably slower, and so to lengthen the day. 

Recently it has been proved that the axis of the 
earth is subject to a very small displacement, or 
rather, ^'wobbling," in the course of a period of 
somewhat over a year. As a consequence of this, 
the pole shifts its place through a circle of, roughly, 
a few yards in width during the time in question. 
This movement is, perhaps, the combined result of 
two causes. One of these is the change of place 
during the year of large masses of material upon our 
earth ; such as occurs, for instance, when ice and snow 

i8o 



The Earth 

melt, or when atmospheric and ocean currents trans- 
port from place to place great bodies of air and water. 
The other cause is supposed to be the fact that the 
earth is not absolutely rigid, and so yields to certain 
strains upon it. In the course of investigation of this 
latter point the interesting conclusion has been reached 
by the famous American astronomer, Professor Simon 
Newcomb, that our globe as a whole is a little more 
rigid than steel. 

We will bring this chapter to a close by alluding 
briefly to two strange appearances which are some- 
times seen in our night skies. These are known re- 
spectively as the Zodiacal Light and the Gegenschein. 

The Zodiacal Light is a faint cone-shaped illu- 
mination which is seen to extend upwards from the 
w^estern horizon after evening twilight has ended, and 
from the eastern horizon before morning twilight has 
begun. It appears to rise into the sky from about the 
position where the sun would be at that time. The 
proper season of th-e year for observing it during the 
evening is in the spring, while in autumn it is best 
seen in the early morning. In our latitudes its light 
is not strong enough to render it visible when the 
moon is full, but in the tropics it is reported to be 
very bright, and easily seen in full moonlight. One 
theory regards it as the reflection of light from swarms 
of meteors revolving round the sun ; another supposes 
it to be a very rarefied extension of the corona. 

The Gegenschein (German for ^' counter-glow ") is a 
faint oval patch of light, seen in the sky exactl}^ op- 
posite to the place of the sun. It is usually treated 
of in connection with the zodiacal light, and one 
theory regards it similarly as of meteoric origin. 

i8i 



Astronomy of To-day 

Another theory, however — that of Mr. Evershed — con- 
siders it a sort of tail to the earth (like a comet's 
tail) composed of hydrogen and helium — the two 
lightest gases we know — driven off from our planet 
in the direction contrary to the sun. 



182 



CHAPTER XVI 

THE MOON 

What we call the moon's '^ phases " are merely the 
various ways in which we see the sun shining upon 
her surface during the course of her monthly re- 
volutions around the earth (see Fig. 14, p. 184). When 
she passes in the neighbourhood of the sun all his 
light falls upon that side which is turned away from 
us, and so the side which is turned towards us is 
unillumined, and therefore invisible. When in this 
position the moon is spoken of as new. 

As she continues her motion around the earth, she 
draws gradually to the east of the sun's place in the 
sky. The sunlight then comes somewhat from the 
side ; and so we see a small portion of the right side 
of the lunar disc illuminated. This is the phase 
known as the crescent moon. 

As she moves on in her orbit more and more of her 
illuminated surface is brought into view ; and so the 
crescent of light becomes broader and broader, until 
we get what is called half-moon, or first quarter, when 
we see exactly one-half of her surface lit up by the 
sun's rays. As she draws still further round yet more 
of her illuminated surface is brought into view, until 
three-quarters of the disc appear lighted up. She is 
then said to be gibbous. 

Eventually she moves round so that she faces the 

183 



Astronomy- of To-day 

sun completely, and the whole of her disc appears 
illuminated. She is then spoken of as full. When 
in this position it is clear that she is on the contrary 

Direction from which the sun's rays are coming. 

J V y I I 






c 



^^ \Earth) ^^1 






.7 



E 
Various positions and illumination of the moon by the sun during her 
revolution around the earth. 



B C D E F G H 

«)Cooa3» _ 

A^ew Crescent First Qua/icr Cibdoui full Cd>6ous Lasf Quarter Crescenl Mew 

The corresponding positions as viewed from the earth, showing the 
consequent phases. 

Fig. 14. — Orbit and Phases of the Moon. 

side of the earth to the sun, and therefore rises about 
the same time that he is setting. She is now, in fact, 
at her furthest from the sun. 

After this, the motion of the moon in her orbit 
carries her on back again in the direction of the sun. 

184 



The Moon 

She thus goes through her phases as before, only 
these of course are in the reverse order. The full 
phase is seen to give place to the gibbous, and this 
in turn to the half-moon and to the crescent ; after 
which her motion carries her into the neighbourhood 
of the sun, and she is once more new, and lost to our 
sight in the solar glare. Following this she draws 
away to the east of the sun again, and the old order 
of phases repeat themselves as before. 

The early Babylonians imagined that the moon had 
a bright and a dark side, and that her phases were 
caused by the bright side coming more and more 
into view during her movement around the sky. The 
Greeks, notably Aristotle, set to work to examine the 
question from a mathematical standpoint, and came 
to the conclusion that the crescent and other appear- 
ances were such as would necessarily result if the 
moon were a dark body of spherical shape illumined 
merely by the light of the sun. 

Although the true explanation of the moon's phases 
has thus been known for centuries, it is unfortunately 
not unusual to see pictures — advertisement posters, for 
instance — in which stars appear within the horns of 
a crescent moon ! Can it be that there are to-day 
educated persons who believe that the moon is a 
thing which grows to a certain size and then wastes 
away again ; who, in fact, do not know that the entire 
body of the moon is there all the while ? 

When the moon shows a very thin crescent, we are 
able dimly to see her still dark portion standing out 
against the sky. This appearance is popularly known 
as the ^^old moon in the new moon's arms." The 
dark part of her surface must, indeed, be to some 

i8s 



Astronomy of To-day 

degree illumined, or we should not be able to see it 
at all. Whence then comes the light which illumines 
it, since it clearly cannot come from the sun ? The 
riddle is easily solved, if we consider what kind of 
view of our earth an observer situated on this 
darkened part of the moon would at that moment get. 
He would, as a matter of fact, just then see nearly 
the whole disc of the earth brightly lit up by sun- 
light. The lunar landscape all around would, there- 
fore, be bathed in what to him would be ^^ earthlight," 
which of course takes the place there of what we call 
moonlight. If, then, we recollect how much greater 
in size the earth is than the moon, it should not 
surprise us that this earthlight will be many times 
brighter than moonlight. It is considered, indeed, to 
be some twenty times brighter. It is thus not at all 
astonishing that we can see the dark portion of the 
moon illumined merely by sunlight reflected upon it 
from our earth. 

The ancients were greatly exercised in their minds 
to account for this '^ earthlight," or ^' earthshine," 
as it is also called. Posidonius (135-51 B.C.) tried to 
explain it by supposing that the moon was partially 
transparent, and that some sunlight consequently 
filtered through from the other side. It was not, 
however, until the fifteenth century that the correct 
solution was arrived at. 

Perhaps the most remarkable thing which one 
notices about the moon is that she always turns the 
same side towards us, and so we never see her other 
side. One might be led from this to jump to the 
conclusion that she does not rotate upon an axis, as 
do the other bodies which we see ; but, paradoxical 

186 



The Moon 

as it may appear, the fact that she turns one face 
always towards the earth, is actually a proof that she 
does rotate upon an axis. The rotation, however, takes 
place with such slowness, that she turns round but 
once during the time in which she revolves around 



lyv 



% 




One side of the moon only is ever presented to the earth. This side 
is here indicated by the letters S.F.E. (side facing earth). 




By placing the above positions in a row, we can see at once that 
the moon makes one complete rotation on her axis in exactly 
the same time as she revolves around the earth. 

Fig. 15 — The Rotation of the Moon on her Axis. 

the earth (see Fig. 15). In order to understand 
the matter clearly, let the reader place an object 
in the centre of a room and walk around it once, 
keeping his face turned towards it the ivhole time, 
While he is doing this, it is evident that he will face 
every one of the four walls of the room in succession. 

187 



Astronomy of To-day 

Now in order to face each of the four walls of a room 
in succession one would be obliged to turn oneself 
entirely round. Therefore, during the act of walking 
round an object with his face turned directly towards 
it, a person at the same time turns his body once 
entirely round. 

In the long, long past the moon must have turned 
round much faster than this. Her rate of rotation 
has no doubt been slowed down by the action of 
some force. It will be recollected how, in the course 
of the previous chapter, we found that the tides were 
tending, though exceedingly gradually, to slow down 
the rotation of the earth upon its axis. But, on 
account of the earth's much greater mass, the force 
of gravitation exercised by it upon the surface of the 
moon is, of course, much more powerful than that 
which the moon exercises upon the surface of the 
earth. The tendency to tidal action on the moon 
itself must, therefore, be much in excess of anything 
which we here experience. It is, in consequence, 
probable that such a tidal drag, extending over a very 
long period of time, has resulted in slowing down the 
moon's rotation to its present rate. 

The fact that we never see but one side of the 
moon has given rise from time to time to fantastic 
speculations with regard to the other side. Some, 
indeed, have wished to imagine that our satellite is 
shaped like an egg, the more pointed end being 
directed away from us. We are here, of course, faced 
with a riddle, which is all the more tantalising from 
its appearing for ever insoluble to men, chained as 
they are to the earth. However, it seems going too 
far to suppose that any abnormal conditions neces- 

i88 



The Moon 

sarily exist at the other side of the moon. As a 
matter of fact, indeed, small portions of that side 
are brought into our view from time to time in con- 
sequence of slight irregularities in the moon's move- 
ment ; and these portions differ in no way from those 
which we ordinarily see. On the whole, we obtain a 
view of about 60 per cent, of the entire lunar surface ; 
that is to say, a good deal more than one-half. 

The actual diameter of the moon is about 2163 
miles, which is somewhat more than one-quarter 
the diameter of the earth. For a satellite, therefore, 
she seems very large compared with her primary, 
the earth ; when we consider that Jupiter's greatest 
satellite, although nearly twice as broad as our moon, 
has a diameter only one twenty-fifth that of Jupiter. 
Furthermore, the moon moves around the earth 
comparatively slowly, making only about thirteen 
revolutions during the entire year. Seen from space, 
therefore, she would not give the impression of a 
circling body, as other satellites do. Her revolutions 
are, indeed, relatively so very slow that she would 
appear rather like a smaller planet accompanying 
the earth in its orbit. In view of all this, some as- 
tronomers are inclined to regard the earth and moon 
rather as a ''double planet" than as a system of 
planet and satellite. 

When the moon is full she attracts more attention 
perhaps than in any of her other phases. The moon, 
in order to be full, must needs be in that region of 
the heavens exactly opposite to the sun. The sun 
appears to go once entirely round the sky in the 
course of a year, and the moon performs the same 
journey in the space of about a month. The moon, 

189 



Astronomy of To-day 

when full, having got half-way round this journey, 
occupies, therefore, that region of the sky which the 
sun itself will occupy half a year later. Thus in 
winter the full moon will be found roughly to occupy 
the sun's summer position in the sky, and in summer 
the sun's winter position. It therefore follows that 
the full moon in winter time is high up in the 
heavens, while in summer time it is low down. We 
thus get the greatest amount of full moonlight when 
it is the most needed. 

The great French astronomer, Laplace, being struck 
by the fact that the ^Messer light" did not rule the 
night to anything like the same extent that the 
" greater light " ruled the day, set to work to examine 
the conditions under which it might have been made 
to do so. The result of his speculations showed 
that if the moon were removed to such a distance 
that she took a year instead of a month to revolve 
around the earth ; and if she were started off in her 
orbit at full moon, she would always continue to 
remain full — a great advantage for us. Whewell, 
however, pointed out that in order to get the moon 
to move with the requisite degree of slowness, she 
would have to revolve so far from the earth that she 
would only look one-sixteenth as large as she does 
at present, which rather militates against the advan- 
tage Laplace had in mind 1 Finally, however, it was 
shown by M. Liouville, in 1845, that the position of 
a perennial full moon, such as Laplace dreamed of, 
would be unstable — that is to say, the body in ques- 
tion could not for long remain undisturbed in the 
situation suggested (see Fig. 16, p. 191). 

There is a well-known phenomenon called harvest 

190 



The Moon 

moon, concerning the nature of which there seems to 
be much popular confusion. An idea in fact appears 
to prevail among a good many people that the moon 

A 

© 



Earth 




^Earff, ^ San^ Earfhi B (J 



Earth 



C 



Various positions of Laplace's " Moon" with regard to the earth 
and sun during the course of a year. 



■©■ 



isf) ^ (^o 

'■■■■©•-''' 
c 

The same positions of Laplace's "Moon," arranged around the earth, 
show that it would make only one revolution in a year. 

Fig, i6. — Laplace's " Perennial Full Moon." 

is a harvest moon when^ at rising, it looks bigger and 
redder than usual. Such an appearance has, how- 
ever, nothing at all to say to the matter ; for ^the 

191 



Astronomy of To-day 

moon always looks larger when low down in the sky, 
and, furthermore, it usually looks red in the later 
months of the year, when there is more mist and fog 
about than there is in summer. What astronomers 
actually term the harvest moon is, indeed, something 
entirely different from this. About the month of 
September the slant at which the full moon comes 
up from below the horizon happens to be such that, 
during several evenings together, she rises almost at 
the same hour, instead of some fifty minutes later, as 
is usually the case. As the harvest is being gathered 
in about that time, it has come to be popularly con- 
sidered that this is a provision of nature, according 
to w^hich the sunlight is, during several evenings, re- 
placed without delay by more or less full-m.oonlight, 
in order that harvesters may continue their work 
straight on into the night, and not be obliged to 
break off after sunset to wait until the moon rises. 
The same phenomenon is almost exactly repeated a 
month later, but by reason of the pursuits then 
carried on it is know^n as the ^^ hunter's moon." 

In this connection should be mentioned that curious 
phenomenon above alluded to, and w^hich seems to 
attract universal notice, namely, that the moon looks 
much larger when near the horizon — at its rising, for 
instance, than when higher up in the sky. This 
seeming enlargement is, however, by no means con- 
fined to the moon. That the sun also looks much 
larger when low down in the sky than when high up, 
seems to strike even the most casual watcher of a 
sunset. The same kind of effect will, indeed, be 
noted if close attention be paid to the stars them- 
selves. A constellation, for instance, appears more 

192 



The Moon 

spread out when low down in the sky than when 
high up. This enlargement of celestial objects 
when in the neighbourhood of the horizon is, how- 
ever, only apparent and not real. It must be entirely 
an illusion; for the most careful measurements of 
the discs of the sun and of the moon fail to show 
that the bodies are any larger when near the 
horizon than when high up in the sky. In fact, if 
there be any difference in measurements with regard 
to the moon, it will be found to be the other way 
round ; for her disc, when carefully measured, is 
actually the slightest degree greater when high in the 
sky, than when low down. The reason for this is 
that, on account of the rotundity of the earth's 
surface, she is a trifle nearer the observer when 
overhead of him. 

This apparent enlargement of celestial objects, when 
low down in the sky, is granted on all sides to be 
an illusion ; but although the question has been dis- 
cussed with animation time out of mind, none of the 
explanations proposed can be said to have received 
unreserved acceptance. The one which usually figures 
in text-books is that we unconsciously compare the 
sun and moon, when low down in the sky, with the 
terrestrial objects in the same field of view, and are 
therefore inclined to exaggerate the size of these orbs. 
Some persons, on the other hand, imagine the illusion 
to have its source in the structure of the human eye ; 
while others, again, put it down to the atmosphere, 
maintaining that the celestial objects in question loom 
large in the thickened air near the horizon, in the 
same way that they do when viewed through fog or 
mist. 

193 N 



Astronomy of To-day 

The writer 1 ventures, however, to think that the 
illusion has its origin in our notion of the shape of 
the celestial vault. One would be inclined, indeed, 
to suppose that this vault ought to appear to us as 
the half of a hollow sphere ; but he maintains that it 
does not so appear, as a consequence of the manner 
in which the eyes of men are set quite close together 
in their heads. If one looks, for instance, high up in 
the sky, the horizon cannot come within the field of 
view, and so there is nothing to make one think that 
the expanse then gazed upon is other than quitejlat — 
let us say like the ceiling of a room. But, as the eyes 
are lowered, a portion of the encircling horizon comes 
gradually into the field of view, and the region of 
the sky then gazed upon tends in consequence to 
assume a hollowed-out form. From this it would seem 
that our idea of the shape of the celestial vault is, 
that it is flattened down over our heads and hollowed 
out all around in the neighbourhood of the horizon (see 
Fig. 17, p. 195). Now, as a consequence of their very 
great distance, all the objects in the heavens neces- 
sarily appear to us to move as if they were placed 
on the background of the vault ; the result being that 
the mind is obliged to conceive them as expanded or 
contracted, in its unconscious attempts to make them 
always fill their due proportion of space in the various 
parts of this abnormally shaped sky. 

From such considerations the writer concludes that 
the apparent enlargement in question is merely the 
natural consequence of the idea we have of the shape 
of the celestial vault — an idea gradually built up in 

'^Journal of the British Astronomical Association^ vol. x. (1899- 1900), 
Nos. I and 3. 

194 



The Moon 

childhood, to become later on what is called '^ second 
nature." And in support of this contention, he would 
point to the fact that the enlargement is not by any 
means confined to the sun and moon, but is every 
whit as marked in the case of the constellations. To 
one who has not noticed this before, it is really quite 
a revelation to compare the appearance of one of the 




A 

The observer is supposed to be standing at A. 

Fig, 17. — Illustrating the author's explanation of the apparent enlarge- 
ment of celestial objects. 

large constellations (Orion, for instance) when high 
up in the sky and when low down. The widening 
apart of the various stars composing the group, when 
in the latter position, is very noticeable indeed. 

Further, if a person were to stand in the centre 
of a large dome, he would be exactly situated as if 
he were beneath the vaulted heaven, and one would 
consequently expect him to suffer the same illusion 
as to the shape of the dome. Objects fixed upon its 
background would therefore appear to him under the 

195 



Astronomy of To-day 

same conditions as objects in the sky, and the illu- 
sions as to their apparent enlargement should hold 
good here also. 

Some years ago a Belgian astronomer, M. Stroobant, 
in an investigation of the matter at issue, chanced to 
make a series of experiments under the very con- 
ditions just detailed. To various portions of the inner 
surface of a large dome he attached pairs of electric 
lights ; and on placing himself at the centre of the 
building, he noticed that, in every case, those pairs 
which were high up appeared closer together than 
those which were low down ! He does not, however, 
seem to have sought for the cause in the vaulted 
expanse. On the contrary, he attributed the effect to 
something connected with our upright stature, to some 
physiological reason which regularly makes us esti- 
mate objects as larger when in front than when 
overhead. 

In connection with this matter, it may be noted 
that it always appears extremely difficult to estimate 
with the eye the exact height above the horizon at 
which any object (say a star) happens to be. Even 
skilled observers find themselves in error in attempt- 
ing to do so. This seems to bear out the writer's 
contention that th^ form under which the celestial 
vault really appears to us is a peculiar one, and tends 
to give rise to false judgments. 

Before leaving this question, it should also be men- 
tioned that nothing perhaps is more deceptive than 
the size which objects in the sky appear to present. 
The full moon looks so like a huge plate, that it 
astonishes one to find that a threepenny bit held at 
arm's length will a long way more than cover its disc. 

196 





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From a photograph taken at the Paris Observatorj' by M. P. 



Paiseux. 

(Page 197) 



The Moon 

The moon is just too far off to allow us to see the 
actual detail on her surface with the naked eye. When 
thus viewed she merely displays a patchy appearance,^ 
and the imaginary forms which her darker markings 
suggest to the fancy are popularly expressed by the 
term '' Man in the Moon." An examination of her 
surface with very moderate optical aid is, however, quite 
a revelation, and the view we then get is not easily 
comparable to what we see with the unaided eye. 

Even with an ordinary opera-glass, an observer 
wall be able to note a good deal of detail upon the 
lunar disc. If it be his first observation of the kind, 
he cannot fail to be struck by the fact to which we 
have just made allusion, namely, the great change 
which the moon appears to undergo when viewed 
with magnifying power. "Cain and his Dog," the 
'' Man in the Moon gathering sticks," or whatever 
indeed his fancy was wont to conjure up from the 
lights and shades upon the shining surface, have now 
completely disappeared ; and he sees instead a silvery 
globe marked here and there with extensive dark 
areas, and pitted all over with crater-like formations 
(see Plate VIII., p. 196). The dark areas retain even 
to the present day their ancient name of '^ seas," for 
Galileo and the early telescopic observers believed 
them to be such, and they are still catalogued under 
the mystic appellations given to them in the long ago ; 
as, for instance, '^ Sea of Showers," " Bay of Rainbows," 
" Lake of Dreams." ^ The improved telescopes of later 

^ Certain of the ancient Greeks thought the markings on the moon to 
be merely the reflection of the seas and lands of our earth, as in a badly 
polished mirror. 

2 Mare Imbrium, Sinus Iridum, Lacus Somniorum. 

197 



Astronomy of To-day 

times showed, however, that they were not really seas 
(there is no w^ater on the moon), but merely areas of 
darker material. 

The crater-like formations above alluded to are the 
"lunar mountains." A person examining the moon 
for the first time with telescopic aid, will perhaps not 
at once grasp the fact that his view of lunar mountains 
must needs be w^hat is called a "bird's-eye" one, 
namely, a view from above, like that from a balloon ; 
and that he cannot, of course, expect to see them 
from the side, as he does the mountains upon the 
earth. But once he has realised this novel point of 
view, he wdll no doubt marvel at the formations which 
lie scattered as it were at his feet. The type of lunar 
mountain is indeed in striking contrast to the terrestrial 
type. On our earth the range-formation is supreme ; 
on the moon the crater-formation is the rule, and is 
so-called from analogy to our volcanoes. A typical 
lunar crater may be described as a circular wall, en- 
closing a central plain, or "floor," which is often 
much depressed below the level of the surface out- 
side. These so-called "craters," or "ring-mountains," 
as they are also termed, are often of gigantic propor- 
tions. For instance, the central plain of one of them, 
known as Ptolemaeus,^ is about 115 miles across, while 
that of Plato is about 60. The walls of craters often 
rise to great heights ; which, in proportion to the 
small size of the moon, are very much in excess of our 
highest terrestrial elevations. Nevertheless, a person 
posted at the centre of one of the larger craters 

^ The lunar craters have, as a rule, received their names from celebrated 
persons, usually men of science. This system of nomenclature was origi- 
nated by Riccioli, in 165 1. 



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Plate IX. Map of the Moon, showing the principal "Craters, 
Mountain- Ranges, and "Seas" 

In this, as in the other plates of the Moon, the South will be found at the top of the 
picture ; such being the view given by the ordinary astronomical telescope, in which all 
objects are seen inverted. 

(Page 199) 



The Moon 

might be surprised to find that he could not see the 
encompassing crater-walls, which would in every 
direction be below his horizon. This would arise not 
alone from the great breadth of the crater itself, but 
also from the fact that the curving of the moon's 
surface is very sharp compared with that of our 
earth. 

We have mentioned Ptolemaeus as among the very 
large craters, or ring-mountains, on the moon. Its 
encompassing walls rise to nearly 13,000 feet, and 
it has the further distinction of being almost in the 
centre of the lunar disc. There are, however, several 
others much wider, but they are by no means in such 
a conspicuous position. For instance, Schickard, 
close to the south-eastern border, is nearly 130 miles 
in diameter, and its wall rises in one point to over 
10,000 feet. Grimaldi, almost exactly at the east 
point, is nearly as large as Schickard. Another 
crater, Clavius, situated near the south point, is about 
140 miles across ; while its neighbour Bailly — named 
after a famous French astronomer of the eighteenth 
century — is 180, and the largest of those which we 
can see (see Plate IX., p. 198). 

Many of the lunar craters encroach upon one 
another ; in fact there is not really room for them all 
upon the visible hemisphere of the moon. About 
30,000 have been mapped ; but this is only a small 
portion, for according to the American astronomer, 
Professor W. H. Pickering, there are more than 
200,000 in all. 

Notwithstanding the fact that the crater is the type 
of mountain associated in the mind with the moon, 
it must not be imagined that upon our satellite there 

199 



Astronomy of To-day 

are no mountains at all of the terrestrial type. There 
are indeed many isolated peaks, but strangely enough 
they are nearly always to be found in the centres of 
craters. Some of these peaks are of great altitude, 
that in the centre of the crater Copernicus being 
over 11,000 feet high. A few mountain ranges also 
exist ; the best known of which are styled, the Lunar 
Alps and Lunar Apennines (see Plate X., p. 200). 

Since the mass of the moon is only about one- 
eightieth that of the earth, it will be understood that 
the force of gravity which she exercises is much 
less. It is calculated that, at her surface, this is 
only about one-sixth of what we experience. A man 
transported to the moon would thus be able to jump 
six times as high as he can here. A building could 
therefore be six times as tall as upon our earth, 
without causing any more strain upon its foundations. 
It should not, then, be any subject for wonder, that 
the highest peaks in the Lunar Apennines attain to 
such heights as 22,000 feet. Such a height, upon 
a comparatively small body like the moon, for her 
volume is only one-fiftieth that of the earth, is rela- 
tively very much in excess of the 29,000 feet of Hima- 
layan structure. Mount Everest, the boast of our planet, 
8000 miles across ! 

High as are the Lunar Apennines, the highest 
peaks on the moon are yet not found among them. 
There is, for instance, on the extreme southern edge 
of the lunar disc, a range known as the Leibnitz 
Mountains ; several peaks of w^hich rise to a height 
of nearly 30,000 feet, one peak in particular being 
said to attain to 36,000 feet (see Plate IX., p. 198). 

But the reader will surely ask the question : 
200 




HE Moon 



We have here (see " Map," Plate IX, p. 198) the mountain ranges of the Apennines, tlie 
Caucasus and the Alps ; also the craters Plato, Aristotle, Eudoxus, C'a-sini. Ai istillus, 
Autolycus, Archimedes and I,inn6. The crater Linne is the very bnuht spi.t in the 
dark area at the upper left liand side of the picture. From a photogr.iph taken at the 
Paris Observatory by M.M. Loewy and I'uiseux. 

(Page 2co) 



The Moon 

" How is it possible to determine the actual height 
of a lunar mountain, if one cannot go upon the moon 
to measure it ? " The answer is, that we can calculate 
its height from noting the length of the shadow 
which it casts. Any one will allow that the length 
of a shadow cast by the sun depends upon two 
things : firstly, upon the height of the object which 
causes the shadow, and secondly, upon the elevation 
of the sun at the moment in the sky. The most 
casual observer of nature upon our earth can scarcely 
have failed to notice that shadows are shortest at 
noon-day, when the sun is at its highest in the sky ; 
and that they lengthen out as the sun declines 
towards its setting. Here, then, we have the clue. 
To ascertain, therefore, the height of a lunar moun- 
tain, we have first to consider at what elevation the 
sun is at that moment above the horizon of the place 
where the mountain in question is situated. Then, 
having measured the actual length in miles of the 
shadow extended before us, all that is left is to 
ask ourselves the question : <^ What height must an 
object be whose shadow cast by the sun, when at 
that elevation in the sky, will extend to this length ? " 

There is no trace whatever of water upon the moon. 
The opinion, indeed, which seems generally held, 
is that water has never existed upon its surface. 
Erosions, sedimentary deposits, and all those marks 
which point to a former occupation by water are 
notably absent. 

Similarly there appears to be no atmosphere on 
the moon ; or, at any rate, such an excessively rare 
one, as to be quite inappreciable. Of this there are 
several proofs. For instance, in a solar eclipse the 

201 



Astronomy of To-day 

moon's disc always stands out quite clear-cut against 
that of the sun. Again during occultations, stars 
disappear behind the moon with a suddenness, which 
could not be the case were there any appreciable 
atmosphere. Lastly, we see no traces of twilight 
upon the lunar surface, nor any softening at the 
edges of shadows ; both which effects would be 
apparent if there were an atmosphere. 

The moon's surface is rough and rocky, and dis- 
plays no marks of the ^^ weathering " that would 
necessarily follow, had it possessed anything Of an 
atmosphere in the past. This makes us rather in- 
clined to doubt that it ever had one at all. Suppos- 
ing, however, that it did possess an atmosphere in 
the 1 past, it is interesting to inquire what may have 
become of it. In the first place it might have gradu- 
ally disappeared, in consequence of the gases w^hich 
composed it uniting chemically with the materials of 
which the lunar body is constructed ; or, again, its 
constituent gases may have escaped into space, in 
accordance with the principles of that kinetic theory 
of which w^e have already spoken. The latter solution 
seems, indeed, the most reasonable of the two, for 
the force of gravity at the lunar surface appears too 
weak to hold down any known gases. This argument 
seems also to dispose of the question of absence of 
water ; for Dr. George Johnstone Stoney, in a careful 
investigation of the subject, has shown that the liquid 
in question, when in the form of vapour, will escape 
from a planet if its mass is less than one-fourth that 
of our earth. And the mass of the moon is very 
much less than this ; indeed only the one-eightiethy as 
we have already stated. 

202 



The Moon 

In consequence of this lack of atmosphere, the 
condition of things upon the moon will be in marked 
contrast to what we experience upon the earth. The 
atmosphere here performs a double service in shield- 
ing us from the direct rays of the sun, and in bottling 
the heat as a glass-house does. On the moon, how- 
ever, the sun beats down in the day-time with a 
merciless force ; but its rays are reflected away from 
the surface as quickly as they are received, and so 
the cold of the lunar night is excessive. It has been 
calculated that the day temperature on the moon may, 
indeed, be as high as our boiling-point, while the 
night temperature may be more than twice as low as 
the greatest cold known in our arctic regions. 

That a certain amount of solar heat is reflected 
to us from the moon is shown by the sharp drop 
in temperature which certain heat-measuring instru- 
ments record when the moon becomes obscured in 
a lunar eclipse. The solar heat which is thus re- 
flected to us by the moon is, however, on the whole 
extremely small ; more light and heat, indeed, reach 
us direct from the sun in half a minute than we g^i 
by reflection from the moon during the entire course 
of the year. 

With regard to the origin of the lunar craters there 
has been much discussion. Some have considered 
them to be evidence of violent volcanic action in the 
dim past ; others, again, as the result of the impact 
of meteorites upon the lunar surface, when the moon 
was still in a plastic condition ; while a third theory 
holds that they were formed by the bursting of huge 
bubbles during the escape into space of gases from 
the interior. The question is, indeed, a very difficult 

203 



Astronomy of To-day 

one. Though volcanic action, such as would result 
in craters of the size of Ptolemaeus, is hard for us to 
picture, and though the lone peaks which adorn the 
centres of many craters have nothing reminiscent of 
them in our terrestrial volcanoes, nevertheless the 
volcanic theory seems to receive more favour than 
the others. 

In addition to the craters there are two more 
features which demand notice, namely, what are 
known as rays and rills. The rays are long, light- 
coloured streaks which radiate from several of the 
large craters, and extend to a distance of some 
hundreds of miles. That they are mere markings on~ 
the surface is proved by the fact that they cast no 
shadows of any kind. One theory is, that they were 
originally great cracks which have been filled with 
lighter coloured material, welling up from beneath. 
The rills, on the other hand, are actually fissures, 
about a mile or so in width and about a quarter of a 
mile in depth. 

The rays are seen to the best advantage in con- 
nection with the craters Tycho and Copernicus 
(see Plate XL, p. 204). In consequence of its fairly 
forward position on the lunar disc, and of the re- 
markable system of rays which issue from it like 
spokes from the axle of a wheel, Tycho commands 
especial attention. The late Rev. T. H. Webb, a 
famous observer, christened it, very happily, the 
^' metropolitan crater of the moon." 

A great deal of attention is, and has been, paid by 
certain astronomers to the moon, in the hope of find- 
ing out if any changes are actually in progress at 
present upon her surface. Sir William Herschel, 

204 



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Plate XL The Moon 

The systems of rays from the craters Tycho, Copernicus and Kepler are well shown here. 
From a photograph taken at the Paris Observatory by ]\1. P. Puiseux-. 

(Page C04) 



# 



The Moon 

indeed, once thought that he saw a lunar volcano in 
eruption, but this proved to be merely the effect of 
the sunlight striking the top of the crater Aristarchus, 
while the region around it was still in shadow — 
sunrise upon Aristarchus, in fact 1 No change of any- 
real importance has, however, been noted, although it 
is suspected that some minor alterations have from 
time to time taken place. For instance, slight varia- 
tions of tint have been noticed in certain areas of the 
lunar surface. Professor W. H. Pickering puts forward 
the conjecture that these may be caused by the growth 
and decay of some low form of vegetation, brought 
into existence by vapours of water, or carbonic acid 
gas, making their way out from the interior through 
cracks near at hand. 

Again, during the last hundred years j one small 
crater known as Linnd (Linnaeus), situated in the 
Mare Serenitatis (Sea of Serenity), has appeared to 
undergo slight changes, and is even said to have 
been invisible for a while (see Plate X,, p. 200). It 
is, however, believed that the changes in question 
may be due to the varying angles at which the sun- 
light falls upon the crater ; for it is an understood 
fact that the irregularities of the moon's motion give 
us views of her surface which; always differ slightly. 

The suggestion has more than once been put for- 
ward that the surface of the moon is covered with 
a thick layer of ice. This is generally considered 
improbable, and consequently the idea has received 
very little support. It first originated with the late 
Mr. S. E. Peal, an English observer of the moon, and 
has recently been resuscitated by the German observer, 
Herr Fauth. 

205 



Astronomy of To-day 

The most unfavourable time for telescopic study 
of the moon is when she is full. The sunlight is then 
falling directly upon her visible hemisphere, and so 
the mountains cast no shadows. We thus do not get 
that impression of hill and hollow which is so very 
noticeable in the other phases. 

The first map of the moon was constructed by 
Galileo. Tobias Mayer published another in 1775 ; 
while during the nineteenth century greatly improved 
ones iwere made by Beer and Madler, Schmidt, 
Neison and others. In 1903, Professor W. H. Picker- 
ing brought out a complete photographic lunar atlas ; 
and a similar publication has recently appeared, the 
work of MM. Loewy and Puiseux of the Observatory 
of Paris. 

The so-called " seas " of the moon are, as we have 
seen, merely dark areas, and there appears to be no 
proof that they were ever occupied by any liquid. 
They are for the most part found in the northern 
portion of the moon ; a striking contrast to our seas 
and oceans, which take up so much of the southern 
hemisphere of the earth. 

There are many erroneous ideas popularly held 
with regard to certain influences which the moon is 
supposed to exercise upon the earth. For instance, 
a change in the weather is widely believed to depend 
upon a change in the moon. But the word '' change " 
as here used is meaningless, for the moon is con- 
tinually changing her phase during the whole of her 
monthly round. Besides, the moon is visible over a 
great portion of the earth at the same moment^ and 
certainly all the places from which it can then be seen 
do not get the same weather ! Further, careful observa- 

206 



The Moon 

tions, and records extending over the past one hundred 
years and more, fail to show any rehable connection 
between the phases of the moon and the condition of 
the weather. 

It has been stated, on very good authority, that no 
telescope ever shows the surface of the moon as 
clearly as we could see it with the naked eye were it 
only 240 miles distant from us. 

Supposing, then, that we were able to approach our 
satellite, and view it without optical aid at such com- 
paratively close quarters, it is interesting to consider 
what would be the smallest detail which our eye 
could take in. The question of the limit of what 
can be appreciated with the naked eye is somewhat 
uncertain, but it appears safe to say that at a distance 
of 240 miles the minutest speck visible would have to 
be at least some 60 yards across. 

Atmosphere and liquid both wanting, the lunar 
surface must be the seat of an eternal calm ; where 
no sound breaks the stillness and where change, as we 
know it, does not exist. The sun beats down upon 
the arid rocks, and inky shadows lie athwart the 
valleys. There is no mellowing of the harsh contrasts. 

We cannot indeed absolutely affirm that Life has 
no place at all upon this airless and waterless globe, 
since we know not under what strange conditions it 
may manifest its presence ; and our most powerful 
telescopes, besides, do not bring the lunar surface 
sufficiently near to us to disprove the existence there 
of even such large creatures as disport themselves 
upon our planet. Still, we find it hard to rid our- 
selves of the feeling that we are in the presence of 
a dead world. On she swings around the earth month 

207 



Astronomy of To-day 

after month, with one face ever turned towards us, 
leaving a certain mystery to hang around that hidden 
side, the greater part of which men can never hope 
to see. The rotation of the moon upon her axis — the 
lunar day — has become, as we have seen, equal to her 
revolution around the earth. An epoch may likewise 
eventually be reached in the history of our own 
planet, when the length of the terrestrial day has been 
so slowed down by tidal friction that it will be equal 
to the year. Then will the earth revolve around the 
central orb, with one side plunged in eternal night and 
the other in eternal sunshine. But such a vista need 
not immediately distress us. It is millions of years 
forward in time. 



208 



CHAPTER XVII 

THE SUPERIOR PLANETS 

♦ 

Having, in a previous chapter, noted the various 

aspects which an inferior planet presents to our view, 

in consequence of its orbit being nearer to the sun 

than the orbit of the earth, it will be well here to 

consider in the same way the case of a superior 

planet, and to mark carefully the difference. 

To begin with, it should be quite evident that we 
cannot ever have a transit of a superior planet. The 
orbit of such a body being entirely outside that of 
the earth, the body itself can, of course, never pass 
between us and the sun. 

A superior planet will be at its greatest distance 
from us when on the far side of the sun. It is said 
then to be in conjunction. As it comes round in its 
orbit it eventually passes, so to speak, at the back 
of us. It is then at its nearest, or in opposition^ as 
this is technically termed, and therefore in the most 
favourable position for telescopic observation of its 
surface. Being, besides, seen by us at that time in 
the direction of the heavens exactly opposite to where 
the sun is, it will thus at midnight be high up in the 
south side of the sky, a further advantage to the 
observer. 

Last of all, a superior planet cannot show crescent 
shapes like an interior ; for whether it be on the far 

209 O 



Astronomy of To-day 

side of the sun, or behind us, or again to our right 
or left, the sunlight must needs appear to fall more 
or less full upon its face. 



The Planetoid Eros 

The nearest to us of the superior planets is the 
tiny body, Eros, which, as has been already stated, 
was discovered so late as the year 1898. In point 
of view, however, of its small size, it can hardly be 
considered as a true planet, and the name '' plane- 
toid " seems much more appropriate to it. 

Eros was not discovered, like Uranus, in the course 
of telescopic examination of the heavens, nor yet, 
like Neptune, as the direct result of difficult calcula- 
tions, but was revealed by the impress of its light 
upon a photographic plate, which had been exposed 
for some length of time to the starry sky. Since 
many of the more recent additions to the asteroids 
have been discovered in the same manner, we shall 
have somewhat more to say about this special em- 
ployment of photography when we come to deal with 
those bodies later on. 

The path of Eros around the sun is so very ellip- 
tical, or, to use the exact technical term, so very 
*' eccentric," that the planetoid does not keep all the 
time entirely in the space between our orbit and that 
of Mars, which latter happens to be the next body 
in the order of planetary succession outwards. In 
portions of its journey Eros, indeed, actually goes 
outside the Martian orbit. The paths of the plane- 
toid and of Mars are, however, not upon the same 
plane, so the bodies always pass clear of each other, 

210 



The Superior Planets 

and there is thus as little chance of their dashing 
together as there would be of trains which run across 
a bridge at an upper level, colliding with those which 
pass beneath it at a lower level. 

When Eros is in opposition, it comes within about 
13J million miles of our earth, and, after the moon, 
is therefore by a long way our nearest neighbour in 
space. It is, however, extremely small, not more, 
perhaps, than twenty miles in diameter, and is subject 
to marked variations in brightness, which do not 
appear up to the present to meet with a satisfactory 
explanation. But, insignificant as is this little body, 
it is of great importance to astronomy ; for it happens 
to furnish the best method known of calculating the 
sun's distance from our earth — a method which Galle, 
in 1872, and Sir David Gill, in 1877, suggested that 
asteroids might be employed for, and which has in 
consequence supplanted the old one founded upon 
transits of Venus. The sun's distance is now an 
ascertained fact to within 100,000 miles, or less than 
half the distance of the moon. 



The Planet Mars 

We next come to the planet Mars. This body 
rotates in a period of slightly more than twenty-four 
hours. The inclination, or slant, of its axis is about 
the same as that of the earth, so that, putting aside 
its greater distance from the sun, the variations of 
season which it experiences ought to be very much 
like ours. 

The first marking detected upon Mars was the 
notable one called the Syrtis Major, also known, on 

211 



Astronomy of To-day 

account of its shape, as the Hour-Glass Sea. This 
observation was made by the famous Huyghens in 
1659; and, from the movement of the marking in 
question across the disc, he inferred that the planet 
rotated on its axis in a period of about twenty-four 
hours. 

There appears to be very little atmosphere upon 
Mars, the result being that we almost always obtain 
a clear view of the detail on its surface. Indeed, it 
is only to be expected from the kinetic theory that 
Mars could not retain much of an atmosphere, as 
the force of gravity at its surface is less than one- 
half of what we experience upon the earth. It should 
here be mentioned that recent researches with the 
spectroscope seem to show that, whatever atmosphere 
there may be upon Mars, its density at the surface of 
the planet cannot be more than the one-fourth part 
of the density of the air at the surface of the earth. 
Professor Lowell, indeed, thinks it may be more 
rarefied than that upon our highest mountain-tops. 

Seen with the naked eye. Mars appears of a red 
colour. Viewed in the telescope, its surface is found 
to be in general of a ruddy hue, varied here and 
there with darker patches of a bluish-green colour. 
These markings are permanent, and were supposed 
by the early telescopic observers to imply a distribu- 
tion of the planet's surface into land and water, the 
ruddy portions being considered as continental areas 
(perhaps sandy deserts), and the bluish-green as seas. 
The similarity to our earth thus suggested was further 
heightened by the fact that broad white caps, situated 
at the poles, were seen to vary with the planet's 
seasons, diminishing greatly ih extent during the 

212 



The Superior Planets 

Martian summer (the southern cap in 1894 even dis- 
appearing altogether), and developing again in the 
Martian winter.^ Readers of Oliver Wendell Holmes 
will no doubt recollect that poet's striking lines : — 

" The snows that glittered on the disc of Mars 
Have melted, and the planet's fiery orb 
Rolls in the crimson summer of its year." 

A state of things so strongly analogous to what 
we experience here, naturally fired the imaginations 
of men, and caused them to look on Mars as a world 
like ours, only upon a much smaller scale. Being 
smaller, it was concluded to have cooled quicker, 
and to be now long past its prime ; and its ^' in- 
habitants" were, therefore, pictured as at a later 
stage of development than the inhabitants of our 
earth. 

Notwithstanding the strong temptation to assume 
that the whiteness of the Martian polar caps is due 
to fallen snow, such a solution is, however, by no 
means so simple as it looks. The deposition of 
water in the form of snow, or even of hoar frost, 
would at least imply that the atmosphere of Mars 
should now and then display traces of aqueous 
vapour, which it does not appear to do.2 It has, 
indeed, been suggested that the whiteness may not 
after all be due to this cause, but to carbonic acid 
gas (carbon dioxide), which is known to freeze at 
a very low temperature. The suggestion is plainly 

^ Sir William Herschel was the first to note these polar changes. 

2 Quite recently, however, Professor Lowell has announced that his 
observer, Mr. E. C. Slipher, finds with the spectroscope faint traces of 
water vapour in the Martian atmosphere. 

213 



Astronomy of To-day 

based upon the assumption that, as Mars is so much 
further from the sun than we are, it would receive 
much less heat, and that the little thus received 
would be quickly radiated away into space through 
lack of atmosphere to bottle it in. 

We now come to those well-known markings, popu- 
larly known as the "canals" of Mars, which have 
been the subject of so much discussion since their 
discovery thirty years ago. 

It was, in fact, in the year 1877, when Mars was 
in opposition, and thus at its nearest to us, that the 
famous Italian astronomer, Schiaparelli, announced 
to the world that he had found that the ruddy areas, 
thought to be continents, were intersected by a net- 
work of straight dark lines. These lines, he reported, 
appeared in many cases to be of great length, so long, 
indeed, as several thousands of miles, and from about 
twenty to sixty miles in width. He christened the 
lines channels^ the Italian word for which, "canali," 
was unfortunately translated into English as " canals." 
The analogy, thus accidentally suggested, gave rise 
to the idea that they might be actual waterways.^ 

In the winter of 1 881-1882, when Mars was again 
in opposition, Schiaparelli further announced that he 
had found some of these lines doubled ; that is to say, 
certain of them were accompanied by similar lines 
running exactly parallel at no great distance away. 
There was at first a good deal of scepticism on the 
subject of Schiaparelli's discoveries, but gradually 
other observers found themselves seeing both the 

^ In a somewhat similar manner the term "crater," as applied to the 
ring-mountain formation on the moon, has evidently given a bias in favour 
of the volcanic theory as an explanation of that peculiar structure. 

214 



The Superior Planets 

lines and their doublings. We have in this a good 
example of a curious circumstance in astronomical 
observation, namely, the fact that when fine detail 
has once been noted by a competent observer, it is 
not long before other observers see the same detail 
with ease. 

An immense amount of close attention has been 
paid to the planet Mars during recent years by the 
American observer, Professor Percival Lowell, at his 
famous observatory, 7300 feet above the sea, near the 
town of Flagstaff, Arizona, U.S.A. His observations 
have not, like those of most astronomers, been con- 
fined merely to *' oppositions," but he has systemati- 
cally kept the planet in view, so far as possible, since 
the year 1894. 

The instrumental equipment of his observatory is 
of the very best, and the " seeing " at Flagstaff is de- 
scribed as excellent. In support of the latter state- 
ment, Mr. Lampland, of the Lowell Observatory, 
maintains that the faintest stars shown on charts made 
at the Lick Observatory with the 36-inch telescope 
there, are perfectly visible with the 24-inch telescope at 
Flagstaff. 

Professor Lowell is, indeed, generally at issue with 
the other observers of Mars. He finds the canals 
extremely narrow and sharply defined, and he attri- 
butes the blurred and hazy appearance, which they 
have presented to other astronomers, to the unsfeady 
and imperfect atmospheric conditions in which their 
observations have been made. He assigns to the 
thinnest a width of two or three miles, and from 
fifteen to twenty to the larger. Relatively to their 
width, however, he finds their length enormous. 

215 



Astronomy of To-day 

Many of them are 2000 miles long, while one is even 
as much as 3540. Such lengths as these are very- 
great in comparison with the smallness of the planet. 
He considers that the canals stand in some peculiar 
relation to the polar cap, for they crowd together in 
its neighbourhood. In place, too, of ill-defined con- 
densations, he sees sharp black spots where the 
canals meet and intersect, and to these he gives the 
name of *' Oases." He further lays particular stress 
upon a dark band of a blue tint, which is always seen 
closely to surround the edges of the polar caps all 
the time that they are disappearing ; and this he takes 
to be a proof that the white material is something 
which actually melts. Of all substances which we know, 
water alone, he affirms, would act in such a manner. 

The question of melting at all may seem strange 
in a planet which is situated so far from the sun, and 
possesses such a rarefied atmosphere. But Professor 
Lowell considers that this very thinness of the atmos- 
phere allows the direct solar rays to fall with great 
intensity upon the planet's surface, and that this 
heating effect is accentuated by the great length of 
the Martian summer. In consequence he concludes 
that, although the general climate of Mars is de- 
cidedly cold, it is above the freezing point of water. 

The observations at Flagstaff appear to do away 
with the old idea that the darkish areas are seas, 
for numerous lines belonging to the so-called ^^ canal 
system" are seen to traverse them. Again, there is 
no star-like image of the sun reflected from them, as 
there would be, of course, from the surface of a great 
sheet of water. Lastly, they are observed to vary 
in tone and colour with the changing Martian seasons, 

216 




S So, 









1) o 



o o o 

J3 - - 

u <u o 






« ™ t« 



The Superior Planets 

the blue-green changing into ochre^ and later on back 
again into blue-green. Professor Lowell regards these 
areas as great tracts of vegetation, which are brought 
into activity as the liquid reaches them from the 
melting snows. 

With respect to the canals, the Lowell observations 
further inform us that these are invisible during the 
Martian winter, but begin to appear in the spring 
when the polar cap is disappearing. Professor 
Lowell, therefore, inclines to the view that in the 
middle of the so-called canals there exist actual water- 
ways which serve the purposes of irrigation, and that 
what we see is not the waterways themselves, for 
they are too narrow, but the fringe of vegetation 
which springs up along the banks as the liquid is 
borne through them from the melting of the polar 
snows. He supports this by his observation that 
the canals begin to appear in the neighbourhood of 
the polar caps, and gradually grow, as it were, in the 
direction of the planet's equator. 

It is the idea of life on Mars which has given this 
planet such a fascination in the eyes of men. A great 
deal of nonsense has, however, been written in news- 
papers upon the subject, and many persons have thus 
been led to think that we have obtained some actual 
evidence of the existence of living beings upon Mars. 
It must be clearly understood, how^ever, that Professor 
Lowell's advocacy of the existence of life upon that 
planet is by no means of this wild order. At the best 
he merely indulges in such theories as his remarkable 
observations naturally call forth. His views are as 
follows : — He considers that the planet has reached 
a time when '' water " has become so scarce that the 

217 



Astronomy of To-day 

^^inhabitants" are obliged to employ their utmost 
skill to make their scanty supply suffice for purposes 
of irrigation. The changes of tone and colour upon 
the Martian surface, as the irrigation produces its 
effects, are similar to what a telescopic observer — say, 
upon Venus — would notice on our earth when the 
harvest ripens over huge tracts of country ; that is, 
of course, if the earth's atmosphere allowed a clear 
view of the terrestrial surface— a very doubtful point 
indeed. Professor Lowell thinks that the perfect 
straightness of the lines, and the geometrical manner 
in which they are arranged, are clear evidences of 
artificiality. On a globe, too, there is plainly no 
reason why the liquid which results from the melting 
of the polar caps should trend at all in the direction 
of the equator. Upon our earth, for instance, the 
transference of water, as in rivers, merely follows the 
slope of the ground, and nothing else. The Lowell 
observations show, however, that the Martian liquid 
is apparently carried from one pole towards the 
equator, and then past it to the other pole, where it 
once more freezes, only to melt again in due season, 
and to reverse the process towards and across the 
equator as before. Professor Lowell therefore holds, 
and it seems a strong point in favour of his theory, 
that the liquid must, in some artificial manner, as by 
pumping, for instance, be helped in its passage across 
the surface of the planet. 

A number of attempts have been made to explain 
the doubling of the canals merely as effects of refrac- 
tion or reflection ; and it has even been suggested 
that it may arise from the telescope not being accu- 
rately focussed. 

218 



The Superior Planets 

The actual doubling of the canals once having been 
doubted, it was an easy step to the casting of doubt 
on the reality of the canals themselves. The idea, 
indeed, was put forward that the human eye, in deal- 
ing with detail so very close to the limit of visibility, 
may unconsciously treat as an actual line several 
point-like markings which merely happen to lie in 
a line. In order to test this theory, experiments 
were carried out in 1902 by Mr. E. W. Maunder of 
Greenwich Observatory, and Mr. J. E. Evans of the 
Royal Hospital School at Greenwich, in which certain 
schoolboys were set to make drawings of a white disc 
with some faint markings upon it. The boys were 
placed at various distances from the disc in question ; 
and it was found that the drawings made by those 
who were just too far off to see distinctly, bore out 
the above theory in a remarkable manner. Recently, 
however, the plausibility of the illusion view has been 
shaken by photographs of Mars taken during the 
opposition of 1905 by Mr. Lampland at the Lowell 
Observatory, in which a number of the more prominent 
canals come out as straight dark lines. Further still, 
in some photographs made there quite lately, several 
canals are said to appear visibly double. 

Following up the idea alluded to in Chapter XVI., 
that the moon may be covered with a layer of ice, 
Mr. W. T. Lynn has recently suggested that this may 
be the case on Mars ; and that, at certain seasons, the 
water may break through along definite lines, and 
even along lines parallel to these. This, he maintains, 
would account for the canals becoming gradually 
visible across the disc, without the necessity of 
Professsor Lowell's ^^ pumping" theory. 
* 219 



Astronomy of To-day 

And now for the views of Professor Lowell himself 
with regard to the doubling of the canals. From his 
observations^ he considers that no pairs of railway- 
lines could apparently be laid down with greater 
parallelism. He draws attention to the fact that the 
doubling does not take place by any means in every 
canal ; indeed, out of 400 canals seen at Flagstaff, 
only fifty-one — or, roughly, one-eighth — have at any 
time been seen double. He lays great stress upon 
this, which he considers points strongly against the 
duplication being an optical phenomenon. He finds 
that the distance separating pairs of canals is much 
less in some doubles than in others, and varies on the 
whole from 75 to 200 miles. According to him, the 
double canals appear to be confined to within 40 
degrees of the equator : or, to quote his own words, 
they are '' an equatorial feature of the planet, confined 
to the tropic and temperate belts." Finally, he points 
out- that they seem to avoid the blue-green areas. 
But, strangely enough, Professor Lowell does not so 
far attempt to fit in the doubling with his body of 
theory. He makes the obvious remark that they may 
be ^' channels and return channels," and with that he 
leaves us. 

The conclusions of Professor Lowell have recently 
been subjected to strenuous criticism by Professor 
W. H. Pickering and Dr. Alfred Russel Wallace. It 
was Professor Pickering who discovered the '^ oases," 
and who originated the idea that we did not see the 
so-called " canals " themselves, but only the growth of 
vegetation along their borders. He holds that the 
oases are craterlets, and that the canals are cracks 
which radiate from them, as do the rifts and streaks 

220 



The Superior Planets 

from craters upon the moon. He goes on to suggest 
that vapours of water, or of carbonic acid gas, escaping 
from the interior, find their way out through these 
cracks, and promote the growth of a low form of 
vegetation on either side of them. In support of this 
view he draws attention to the existence of long 
"steam-cracks," bordered by vegetation, in the deserts 
of the highly volcanic island of Hawaii. We have 
already seen, in an earlier chapter, how he has applied 
this idea to the explanation of certain changes which 
are suspected to be taking place upon the moon. 

In dealing with the Lowell canal system. Professor 
Pickering points out that under such a slight atmos- 
pheric pressure as exists on Mars, the evaporation 
of the polar caps — supposing them to be formed of 
snow — would take place with such extraordinary 
rapidity that the resulting water could never be made 
to travel along open channels, but that a system of 
gigantic tubes or water-mains would have to be 
employed ! 

As will be gathered from his theories regarding 
vegetation, Professor Pickering does not deny the 
existence of a form of life upon Mars. But he will 
not hear of civilisation, or of anything even approach- 
ing it. He thinks, however, that as Mars is inter- 
mediate physically between the moon and earth, the 
form of life which it supports may be higher than that 
on the moon and lower than that on the earth. 

In a small book published in the latter part of 1907, 
and entitled Is Mars Habitable? Dr. Alfred Russel 
Wallace sets himself, among other things, to combat 
the idea of a comparatively high temperature, such 
as Professor Lowell has allotted to Mars. He shows 

221 



Astronomy of To-day 

the immense service which the water-vapour in our 
aknosphere exercises, through keeping the solar heat 
from escaping from the earth's surface. He then 
draws attention to the fact that there is no spectro- 
scopic evidence of water-vapour on Mars ^ ; and points 
out that its absence is only to be expected, as Dr. 
George Johnstone Stoney has shown that it will 
escape from a body whose mass is less than one- 
quarter the mass of the earth. The mass of Mars 
is, in fact, much less than this, i.e. only one-ninth. 
Dr. Wallace considers, therefore, that the temperature 
of Mars ought to be extremely low, unless the con- 
stitution of its atmosphere is very different from ours. 
With regard to the latter statement, it should be 
mentioned that the Swedish physicist, Arrhenius, has 
recently shown that the carbonic acid gas in our 
atmosphere has an important influence upon climate. 
The amount of it in our air is, as we have seen, 
extremely small ; but Arrhenius shows that, if it were 
doubled, the temperature would be more uniform and 
much higher. We thus see how futile it is, with our 
present knowledge, to dogmatise on the existence or 
non-existence of life in other celestial orbs. 

As to the canals Dr. Wallace puts forward a 
theory of his own. He contends that after Mars 
had cooled to a state of solidity, a great swarm of 
meteorites and small asteroids fell in upon it, with 
the result that a thin molten layer was formed all 
over the planet. As this layer cooled, the imprisoned 
gases escaped, producing vents or craterlets ; and as 
it attempted to contract further upon the solid interior, 
it split in fissures radiating from points of weakness, 

1 Mr. Slipher's results (see note 2, page 213) were not then known. 
222 



The Superior Planets 

such; for instance, as the craterlets. And he goes 
on to suggest that the two tiny Martian satellites, 
with which we shall deal next, are the last survivors 
of his hypothetical swarm. Finally, with regard to 
the habitability of Mars, Dr. Wallace not only denies 
it, but asserts that the planet is ^^ absolutely unin- 
habitable." 

For a long time it was supposed that Mars did not 
possess any satellites. In 1877, however, during that 
famous opposition in which Schiaparelli first saw the 
canals, two tiny satellites were discovered at the 
Washington Observatory by an American astronomer, 
Professor Asaph Hall. These satellites are so minute, 
and so near to the planet, that they can only be 
seen with very large telescopes ; and even then the 
bright disc of the planet must be shielded off. They 
have been christened Phobos and Deimos (Fear and 
Dread) ; these being the names of the two subordi- 
nate deities who, according to Homer, attended upon 
Mars, the god of war. 

It is impossible to measure the exact sizes of these 
satellites, as they are too small to show any discs, 
but an estimate has been formed from their bright- 
ness. The diameter of Phobos was at first thought 
to be six miles, and that of Deimos, seven. As later 
estimates, however, considerably exceed this, it will, 
perhaps, be not far from the truth to state that they 
are each roughly about the size of the planetoid Eros. 
Phobos revolves around Mars in about 7J hours, at 
a distance of about only 4000 miles from the planet's 
surface, and Deimos in about 30 hours, at a distance 
of about 12,000 miles. As Mars rotates on its axis 
in about 24 hours, it will be seen that Phobos makes 

223 



Astronomy of To-day 

more than three revolutions while the planet is 
rotating once — a very interesting condition of things. 

A strange foreshadowing of the discovery of the 
satellites of Mars will be familiar to readers of 
Gulliver's Travels. According to Dean Swift's hero, 
the astronomers on the Flying Island of Laputa had 
found two tiny satellites to Mars, one of which re- 
volved around the planet in ten hours. The correct- 
ness of this guess is extraordinarily close, though 
at best it is, of course, nothing more than a pure 
coincidence. 

It need not be at all surprising that much uncertainty 
should exist with regard to the actual condition of 
the surface of Mars. The circumstances in which we 
are able to see that planet at the best are, indeed, 
hardly sufficient to warrant us in propounding any 
hard and fast theories. One of the most experienced 
of living observers, the American astronomer. Pro- 
fessor E. E. Barnard, considers that the view we 
get of Mars with the best telescope may be fairly 
compared with our naked eye view of the moon. 
Since we have seen that a view with quite a small 
telescope entirely alters our original idea of the lunar 
surface, a slight magnification revealing features of 
whose existence we had not previously the slightest 
conception, it does not seem too much to say that a 
further improvement in optical power might entirely 
subvert the present notions with regard to the Martian 
canals. Therefore, until we get a still nearer view 
of these strange markings, it seems somewhat futile 
to theorise. The lines which we see are perhaps, 
indeed, a foreshortened and all too dim view of some 
type of formation entirely novel to us, and possibly 

224 



The Superior Planets 

peculiar to Mars. Differences of gravity and other 
conditions, such as obtain upon different planets, may 
perhaps produce very diverse results. The earth, 
the moon, and Mars differ greatly from one another 
in size, gravitation, and other such characteristics. 
Mountain-ranges so far appear typical of our globe, 
and ring-mountains typical of the moon. May not 
the so-called *' canals " be merely some special forma- 
tion peculiar to Mars, though quite a natural result 
of its particular conditions and of its past history ? 



The Asteroids (or Minor Planets) 

We now come to that belt of small planets which 
are known by the name of asteroids. In the general 
survey of the solar system given in Chapter II., we 
saw how it was long ago noticed that the distances of 
the planetary [orbits from the sun would have pre- 
sented a marked appearance of orderly sequence, 
were it not for a gap between the orbits of Mars 
and Jupiter where no large planet was known to 
circulate. The suspicion thus aroused that some 
planet might, after all, be moving in this seemingly 
empty space, gave rise to the gradual discovery of a 
great number of small bodies ; the largest of which, 
Ceres, is less than 500 miles in diameter. Up to the 
present day some 600 of these bodies have been 
discovered ; the four leading ones, in order of size, 
being named Ceres, Pallas, Juno, and Vesta. All the 
asteroids are invisible to the naked eye, with the 
exception of Vesta, which, though by no means the 
largest, happens to be the brightest. It is, however, 
only just visible to the eye under favourable condi- 

225 p 



Astronomy of To-day 

tions. No trace of an atmosphere has been noted 
upon any of the asteroids, but such a state of thmgs 
is only to be expected from the kinetic theory. 

For a good many years the discoveries of asteroids 
were made by means of the telescope. When, in the 
course of searching the heavens, an object was noticed 
which did not appear upon any of the recognised 
star charts, it was kept mider observation for several 
nights to see whether it changed its place in the sky. 
Since asteroids move around the sun in orbits, just as 
planets do, they, of course, quickly reveal themselves 
by their change of position against the starry back- 
ground. 

The year 1891 started a new era in the discovery 
of asteroids. It occurred to the Heidelberg observer, 
Dr. Max Wolf, one of the most famous of the hunters 
of these tiny planets, that photography might be 
employed in the quest with success. This photo- 
graphic method, to which allusion has already been 
made in dealing with Eros, is an extremely simple 
one. If a photograph of a portion of the heavens be 
taken through an ^'equatorial" — that is, a telescope, 
moving by machinery, so as to keep the stars, at 
which it is pointed, always exactly in the field of view 
during their apparent movement across the sky — the 
images of these stars will naturally come out in the 
photograph as sharply defined points. If, however, 
there happens to be an asteroid, or other planetary 
body, in the same field of view, its image will come 
out as a short white streak ; because the body has 
a comparatively rapid motion of its own, and will, 
during the period of exposure, have moved sufficiently 
against the background of the stars to leave a short 

226 



The Superior Planets 

trail, instead of a dot, upon the photographic plate. 
By this method Wolf himself has succeeded in dis- 
covering more than a hundred asteroids (see Plate 
XIII., p. 226). It was, indeed, a little streak of this 
kind, appearing upon a photograph taken by the 
astronomer Witt, at Berlin, in 1898, which first 
informed the world of the existence of Eros. 

It has been calculated that the total mass of the 
asteroids must be much less than one-quarter that of 
the earth. They circulate as a rule within a space of 
some 30,000,000 miles in breadth, lying about midway 
between the paths of Mars and Jupiter. Two or three, 
however, of the most recently discovered of these 
small bodies have been found to pass quite close to 
Jupiter. The orbits of the asteroids are by no means 
in the one plane, that of Pallas being the most inclined 
to the plane of the earth's orbit. It is actually three 
times as much inclined as that of Eros. 

Two notable theories have been put forward to 
account for the origin of the asteroids. The first is 
that of the celebrated German astronomer, Olbers, 
who was the discoverer of Pallas and Vesta. He 
suggested that they were the fragments of an ex- 
ploded planet. This theory was for a time generally 
accepted, but has now been abandoned in consequence 
of certain definite objections. The most important 
of these objections is that, in accordance with the 
theory of gravitation, the orbits of such fragments 
would all have to pass through the place where the 
explosion originally occurred. But the wide area 
over which the asteroids are spread points rather 
against the notion that they all set out originally from 
one particular spot. Another objection is that it does 

227 



Astronomy of To-day 

not appear possible that, within a planet already 
formed; forces could originate sufficiently powerful to 
tear the body asunder. 

The second theory is that for some reason a planet 
here failed in the making. Possibly the powerful^ 
gravitational action of the huge body of Jupiter hard 
by, disturbed this region so much that the matter 
distributed through it was never able to collect itself 
into a single mass. 



228 



CHAPTER XVIII 

THE SUPERIOR FL Al<iET S~continued 

The planets, so far, have been divided into inferior 
and superior. Such a division, however, refers 
merely to the situation of their orbits with regard to 
that of our earth. There is, indeed, another manner 
in which they are often classed, namely, according 
to size. On this principle they are divided into two 
groups ; one group called the Terrestrial Planets, or 
those which have characteristics like our earth, and 
the other called the Major Planets^ because they are 
all of very great size. The terrestrial planets are 
Mercury, Venus, the earth, and Mars. The major 
planets are the remainder, namely, Jupiter, Saturn, 
Uranus, and Neptune. As the earth's orbit is the 
boundary which separates the inferior from the 
superior planets, so does the asteroidal belt divide 
the terrestrial from the major planets. We found 
the division into inferior and superior useful for 
emphasising the marked difference in aspect which 
those two classes present as seen from our earth ; 
the inferior planets showing phases like the moon 
when viewed in the telescope, whereas the superior 
planets do not. But the division into terrestrial and 
major planets is the more far-reaching classification 
of the two, for it includes the whole number of 
planets, whereas the other arrangement necessarily 

229 



Astronomy of To-day 

excludes the earth. The members of each of these 
classes have many definite characteristics in common. 
The terrestrial planets are all of them relatively small 
in size, comparatively near together, and have few or 
no satellites. They are, moreover, rather dense in 
structure. The major planets, on the other hand, are 
huge bodies, circulating at great distances from each 
other, and are, as a rule, provided with a number of 
satellites. With respect to structure, they may be 
fairly described as being loosely put together. Further, 
the markings on the surfaces of the terrestrial planets 
are permanent, whereas those on the major planets 
are continually shifting. 



The Planet Jupiter 

Jupiter is the greatest of the major planets. It has 
been justly called the '' Giant " planet, for both in 
volume and in mass it exceeds all the other planets 
put together. When seen through the telescope it 
exhibits a surface plentifully covered with markings, 
the most remarkable being a series of broad parallel 
belts. The chief belt lies in the central parts of the 
planet, and is at present about 10,000 miles wide. It 
is bounded on either side by a reddish brown belt 
of about the same width. Bright spots also appear 
upon the surface of the planet, last for a w^hile, and 
then disappear. The most notable of the latter is one 
known as the ^' Great Red Spot." This is situated 
a little beneath the southern red belt, and appeared 
for the first time about thirty years ago. It has 
undergone a good many changes in colour and bright- 
ness, and is still faintly visible. This spot is the 

230 



The Superior Planets 

most permanent marking which has yet been seen upon 
Jupiter. In general, the markings change so often that 
the surface which we see is evidently not solid, but of a 
fleeting nature akin to cloud (see Plate XIV., p. 230). 

Observations of Jupiter's markings show that on an 
average the planet rotates on its axis in a period of 
about 9 hours 54 minutes. The mention here of an 
average with reference to the rotation will, no doubt, 
recall to the reader's mind the similar case of the 
sun, the different portions of which rotate with 
different velocities. The parts of Jupiter which 
move quickest take 9 hours 50 minutes to go round, 
while those which move slowest take 9 hours 57 
minutes. The middle portions rotate the fastest, a 
phenomenon which the reader will recollect was also 
the case with regard to the sun. 

Jupiter is a very loosely packed body. Its density 
is on an average only about ij times that of water, 
or about one-fourth the density of the earth ; but its 
bulk is so great that the gravitation at that surface 
which we see is about 2J times what it is on the 
surface of the earth. In accordance, therefore, with 
the kinetic theory, we may expect the planet to 
retain an extensive layer of gases around it ; and 
this is confirmed by the spectroscope, which gives 
evidence of the presence of a dense atmosphere. 

All things considered, it may be safely inferred that 
the interior of Jupiter is very hot, and that what we 
call its surface is not the actual body of the planet, but 
a voluminous layer of clouds and vapours driven up- 
wards from the heated mass underneath. The planet 
was indeed formerly thought to be self-luminous ; but 
this can hardly be the case, for those portions of the 

231 



Astronomy of To-day 

surface which happen to He at any moment in the 
shadows cast by the satellites appear to be quite black. 
Again, when a satellite passes into the great shadow 
cast by the planet it becomes entirely invisible, which 
would not be the case did the planet emit any per- 
ceptible light of its own. 

In its revolutions around the sun, Jupiter is attended, 
so far as we know, by seven ^ satellites. Four of these 
were among the first celestial objects which Galileo dis- 
covered with his "optick tube," and he named them 
the ^^ Medicean Stars " in honour of his patron, Cosmo 
de Medici. Being comparatively large bodies they 
might indeed just be seen with the naked eye, were 
it not for the overpowering glare of the planet. 

It was only in quite recent times, namely, in 
1892, that a fifth satellite was added to the system 
of Jupiter. This body, discovered by Professor E. E. 
Barnard, is very small. It circulates nearer to the 
planet than the innermost of Galileo's moons ; and, 
on account of the glare, is a most difficult object to 
obtain a glimpse of, even in the best of telescopes. 
In December 1904 and January 1905 respectively, two 
more moons were added to the system, these being 
found by photography ^ by the American astronomer, 
Professor C. D. Perrine. Both the bodies in question 
revolve at a greater distance from the planet than the 
outermost of the older known satellites. 

^ Mr. P. Melotte, of Greenwich Observatory, while examining a photo- 
graph taken there on Februaiy 28, 1908, discovered upon it a very faint 
object which it is firmly believed will prove to be an eighth satellite of 
Jupiter. This object was afterwards found on plates exposed as far back 
as January 27. It has since been photographed several times at Greenwich, 
and also at Heidelberg (by Dr. Max Wolf) and at the Lick Observatory. 
Its movement is probably retrograde, like that of Phoebe (p. 240). 

232 



The Superior Planets 

Galileo's moons, though the largest bodies of 
Jupiter's satellite system, are, as we have already 
pointed out, very small indeed when compared with 
the planet itself. The diameters of two of them, 
Europa and lo, are, however, about the same as that 
of our moon, while those of the other two, Callisto 
and Ganymede, are more than half as large again. 
The recently discovered satellites are, on the other 
hand, insignificant ; that found by Barnard, for 
example, being only about loo miles in diameter. 

Of the four original satellites lo is the nearest to 
Jupiter, and, seen from the planet, it would show 
a disc somewhat larger than that of our moon. The 
others would appear somewhat smaller. However, 
on account of the great distance of the sun, the 
entire light reflected to Jupiter by all the satellites 
should be very much less than what we get from our 
moon. 

Barnard's satellite circles around Jupiter at a dis- 
tance less than our moon is from us, and in a period 
of about 12 hours. Galileo's four satellites revolve in 
periods of about 2, 2>i) 7? ^nd 16^ days respectively, at 
distances lying roughly between a quarter of a million 
and one million miles. Perrine's two satellites are at 
a distance of about seven million miles, and take 
about nine months to complete their revolutions. 

The larger satellites, when viewed in the telescope, 
exhibit certain defined markings ; but the bodies are 
so far away from us, that only those details which are 
of great extent can be seen. The satellite lo, accord- 
ing to Professor Barnard, shows a darkish disc, with 
a broad white belt across its middle regions. Mr. 
Douglass, one of the observers at the Lowell Obser- 

233 



Astronomy of To-day 

vatory, has noted upon Ganymede a number of 
markings somewhat resembHng those seen on Mars, 
and he concludes, from their movement, that this 
satelHte rotates on its axis in about seven days. Pro- 
fessor Barnard, on the other hand, does not corro- 
borate this, though he claims to have discovered 
bright polar caps on both Ganymede and Callisto. 

In an earlier chapter we dealt at length with 
eclipses, occultations, and transits, and endeavoured 
to make clear the distinction between them. The 
system of Jupiter's satellites furnishes excellent ex- 
amples of all these phenomena. The planet casts a 
very extensive shadow, and the satellites are con- 
stantly undergoing obscuration by passing through it. 
Such occurrences are plainly comparable to our lunar 
eclipses. Again, the satellites may, at one time, be 
occulted by the huge disc of the planet, and at 
another time seen in transit over its face. A fourth 
phenomenon is what is known as an eclipse of the 
planet by a satellite, which is the exact equivalent of 
what we style on the earth an eclipse of the sun. In 
this last case the shadow, cast by the satellite, appears 
as a round black spot in movement across the planet's 
surface. 

In the passages of these attendant bodies behind 
the planet, into its shadow, or across its face, respec- 
tively, it occasionally happens that Galileo's four 
satellites all disappear from view, and the planet is 
then seen for a while in the unusual condition of 
being apparently without its customary attendants. 
An instance of this phenomenon took place on the 
3rd of October 1907. On that occasion, the satellites 
known as I. and III. (i,e. lo and Ganymede) were 

234 



The Superior Planets 

eclipsed, that is to say, obscured by passing into the 
planet's shadow ; Satellite IV. (Callisto) was occulted 
by the planet's disc ; while Satellite II. (Europa), being 
at the same moment in transit across the planet's face, 
was invisible against that brilliant background. A 
number of instances of this kind of occurrence are on 
record. Galileo, for example, noted one on the 15th 
of March 161 1, while Herschel observed another on 
the 23rd of May 1802. 

It was indirectly to Jupiter's satellites that the world 
was first indebted for its knowledge of the velocity 
of light. When the periods of revolution of the 
satellites were originally determined, Jupiter happened, 
at the time, to be at his nearest to us. From the 
periods thus found tables were made for the pre- 
diction of the moments at which the eclipses and 
other phenomena of the satellites should take place. 
As Jupiter, in the course of his orbit, drew further 
away from the earth, it was noticed that the dis- 
appearances of the satellites into the shadow of the 
planet occurred regularly later than the time pre- 
dicted. In the year 1675, Roemer, a Danish astro- 
nomer, inferred from this, not that the predictions were 
faulty, but that light did not travel instantaneously. 
It appeared, in fact, to take longer to reach us, the 
greater the distance it had to traverse. Thus, when 
the planet was far from the earth, the last ray given 
out by the satellite, before its passage into the shadow, 
took a longer time to cross the intervening space, 
than when the planet was near. Modern experiments 
in physics have quite confirmed this, and have proved 
for us that light does not travel across space in the 
twinkling of an eye, as might hastily be supposed, 

235 



Astronomy of To-day 

but actually moves, as has been already stated, at the 
rate of about 186,000 miles per second. 



The Planet Saturn 

Seen in the telescope the planet Saturn is a wonder- 
ful and very beautiful object. It is distinguished from 
all the other planets, in fact from all known celestial 
bodies, through being girt around its equator by 
what looks like a broad, flat ring of exceeding thinness. 
This, however, upon closer examination, is found to 
be actually composed of three concentric rings. The 
outermost of these is nearly of the same brightness 
as the body of the planet itself. The ring which comes 
immediately within it is also bright, and is separated 
from the outer one all the way round by a relatively 
narrow space, known as " Cassini's division," because 
it was discovered by the celebrated French astro- 
nomer, ]. D. Cassini, in the year 1675. Inside the 
second ring, and merging insensibly into it, is a third 
one, known as the " crape ring," because it is darker 
in hue than the others and partly transparent, the 
body of Saturn being visible through it. The inner 
boundary of this third and last ring does not adjoin 
the planet, but is everywhere separated from it by 
a definite space. This ring was discovered inde- 
pendently^ in 1850 by Bond in America and Dawes 
in England. 

^ In the history of astronomy two salient points stand out. 

The first of these is the number of " independent " discoveries which 
have taken place ; such, for instance, as in the cases of Le Verrier and 
Adams with regard to Neptune, and of Lockyer and Janssen in the matter 
of the spectroscopic method of observing solar prominences. 

The other is the great amount of "anticipation." Copernicus, as we 

236 




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<u 


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H ^ « y 






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lit 



The Superior Planets 

As distinguished from the crape ring, the bright 
rings must have a considerable closeness of texture ; 
for the shadow of the planet may be seen projected 
upon them, and their shadows in turn projected upon 
the surface of the planet (see Plate XV., p. 236). 

According to Professor Barnard, the entire breadth 
of the ring system, that is to say, from one side to the 
other of the outer ring, is 172,310 miles, or some- 
what more than double the planet's diameter. 

In the varying views which we get of Saturn, the 
system of the rings is presented to us at very different 
angles. Sometimes we are enabled to gaze upon its 
broad expanse ; at other times, however, its thin edge 
is turned exactly towards us, an occurrence which 
takes place after intervals of about fifteen years. When 
this happened in 1892 the rings are said to have 
disappeared entirely from view in the great Lick 
telescope. We thus get an idea of their small degree 
of thickness, which would appear to be only about 
50 miles. The last time the system of rings was 
exactly edgewise to the earth was on the 3rd of 
October 1907. 

The question of the composition of these rings has 
given rise to a good deal of speculation. It was 
formerly supposed that they were either solid or liquid, 
but in 1857 it was proved by Clerk Maxwell that a 
structure of this kind would not be able to stand. 
He showed, however, that they could be fully explained 
by supposing them to consist of an immense number of 

have seen, was anticipated by the Greeks ; Kepler was not actually the 
first who thought of elliptic orbits ; others before Newton had imagined an 
attractive force. 

Both these points furnish much food for thought ! 



Astronomy of To-day 

separate solid particles, or, as one might otherwise put 
it, extremely small satellites, cirding in dense swarms 
around the middle portions of the planet. It is there- 
fore believed that we have here the materials ready 
for the 'formation of a satellite or satellites ; but that 
the powerful gravitative action, arising through the 
planet's being so near at hand, is too great ever to 
allow these materials to aggregate themselves into a 
solid mass. There is, as a matter of fact, a mini- 
mum distance from the body of any planet within 
which it can be shown that a satellite will be unable 
to form on account of gravitational stress. This is 
known as ^' Roche's limit," from the name of a 
French astronomer who specially investigated the 
question. 

There thus appears to be a certain degree of 
analogy between Saturn's rings and the asteroids. 
Empty spaces, too, exist in the asteroidal zone, the 
relative position of one of which bears a striking 
resemblance to that of ^'Cassini's division." It is 
suggested, indeed, that this division had its origin in 
gravitational disturbances produced by the attraction 
of the larger satellites, just as the empty spaces in the 
asteroidal zone are supposed to be the result of per- 
turbations caused by the Giant Planet hard by. 

It has long been understood that the system of the 
rings must be rotating around Saturn, for if they were 
not in motion his intense gravitational attraction 
would quickly tear them in pieces. This was at 
length proved to be the fact by the late Professor 
Keeler, Director of the Lick Observatory, who from 
spectroscopic observations found that those portions 
of the rings situated near to the planet rotated faster 

238 



The Superior Planets 

than those farther from it. This directly supports 
the view that the rings are composed of satellites ; for, 
as we have already seen, the nearer a satellite is to its 
primary the faster it will-revolve. On the other hand, 
were the rings solid, their outer portions would move 
the fastest; as we have seen takes place in the body of 
the earth, for example. The mass of the ring system, 
however, must be exceedingly small, for it does not 
appear to produce any disturbances in the movements 
of Saturn's satellites. From the kinetic theory, there- 
fore, one would not expect to find any atmosphere on 
the rings, and the absence of it is duly shown by 
spectroscopic observations. 

The diameter of Saturn, roughly speaking, is about 
one-fifth less than that of Jupiter. The planet is very 
flattened at the poles, this flattening being quite 
noticeable in a good telescope. For instance, the 
diameter across the equator is about 76,470 miles, 
while from pole to pole it is much less, namely, 69,770. 

The surface of Saturn bears a strong resemblance 
to that of Jupiter. Its markings, though not so well 
defined, are of the same belt-like description ; and 
from observation of them it appears that the planet 
rotates on an average in a little over ten hours. The 
rotation is in fact of the same peculiar kind as that 
of the sun and Jupiter ; but the difference of speed at 
which the various portions of Saturn go round are 
even more marked than in the case of the Giant 
Planet. The density of Saturn is less than that of 
Jupiter ; so that it must be largely in a condition of 
vapour, and in all probability at a still earlier stage of 
planetary evolution. 

Up to the present we know of as many as ten 

239 



Astronomy of To-day 

satellites circling around Saturn, which is more than 
any other planet of the solar system can lay claim to. 
Two of these, however, are very recent discoveries ; 
one, Phoebe, having been found by photography in 
August 1898, and the other, Themis, in 1904, also 
by the same means. For both of these we are in- 
debted to Professor W. H. Pickering. Themis is 
said to be the faintest object in the solar system. It 
cannot be seen^ even with the ^argest telescope in 
existence ; a fact which should hardly fail to impress 
upon one the great advantage the photographic plate 
possesses in these researches over the human eye. 

The most important of the whole Saturnian family 
of satellites are the two known as Titan and japetus. 
These were discovered respectively by Huyghens in 
1655 and by Cassini in 1671. Japetus is about the 
same size as our moon ; - while the diameter of Titan, 
the largest of the satellites, is about half as much 
again. Titan takes about sixteen days to revolve 
around Saturn, while Japetus takes more than two 
months and a half. The former is about three-quarters 
of a million miles distant from the planet, and the 
latter about two and a quarter millions. To Sir 
William Herschel we are indebted for the discovery 
of two more satellites, one of which he found on the 
evening that he used his celebrated 40-foot telescope 
for the first time. The ninth satellite, Phoebe, one 
of the two discovered by Professor Pickering, is 
perhaps the most remarkable body in the solar 
system, for all the other known members of that 
system perform their revolutions in one fixed direc- 
tion, whereas this satellite revolves in the contrary 
direction. 

240 



The Superior Planets 

In consequence of the great distance of Saturn, the 
sun, as seen from the planet, would appear so small 
that it would scarcely show any disc. The planet, 
indeed, only receives from the sun about one-nine- 
tieth of the heat and light which the earth receives. 
Owing to this diminished intensity of illumination, 
the combined light reflected to Saturn by the whole 
of its satellites must be very small. 

With the sole exception of Jupiter, not one of the 
planets circulating nearer to the sun could be seen 
from Saturn, as they would be entirely lost in the 
solar glare. For an observer upon Saturn, Jupiter 
would, therefore, fill much the same position as Venus 
does for us, regularly displaying phases and being 
alternately a morning and an evening star. 

It is rather interesting to consider the appearances 
which would be produced in our skies were the earth 
embellished with a system of rings similar to those 
of Saturn, In consequence of the curving of the 
terrestrial surface, they would not be seen at all from 
within the Arctic or Antarctic circles, as they would 
be always below the horizon. From the equator they 
would be continually seen edgewise, and so would 
appear merely as line of light stretching right across 
the heaven and passing through the zenith. But the 
dwellers in the remaining regions would find them 
very objectionable, for they would cut off the light- 
of the sun during lengthy periods of time. 

Saturn was a sore puzzle to the early telescopic 
observers. They did not for a long time grasp the 
fact that it was surrounded by a ring — so slow is 
the human mind to seek for explanations out of the 
ordinary course of things. The protrusions of the 

241 Q 



Astronomy of To-day 

ring on either side of the planet, at first looked to 
Galileo like two minor globes placed on opposite 
sides of it; and slightly overlapping the disc. He 
therefore informed Kepler that ^^ Saturn consists of 
three stars in contact with one another." Yet he was 
genuinely puzzled by the fact that the two attendant 
bodies (as he thought them) always retained the same 
position with regard to the planet's disc, and did not 
appear to revolve around it, nor to be in any wise 
shifted as a consequence of the movements of our 
earth. 

About a year and a half elapsed before he again 
examined Saturn ; and, if he was previously puzzled, 
he was now thoroughly amazed. It happened just 
then to be one of those periods when the ring is 
edgewise towards the earth, and of course he only 
saw a round disc like that of Jupiter. What, indeed, 
had become of the attendant orbs ? Was some demon 
mocking him ? Had Saturn devoured his own chil- 
dren ? He was, however, fated to be still more puzzled, 
for soon the minor orbs reappeared, and, becoming 
larger and larger as time went on, they ended by 
losing their globular appearance and became like 
two pairs of arms clasping the planet from each 
side ! (see Plate XVI., p. 242). 

Galileo went to his grave with the riddle still un- 
solved, and it remained for the famous Dutch astro- 
nomer, Huyghens, to clear up the matter. It was, 
however, some little time before he hit upon the real 
explanation. Having noticed that there were dark 
spaces between the strange appendages and the body 
of the planet, he imagined Saturn to be a globe fitted 
with handles at each side; "ansae" these came to be 

242 



:<iiiiiiiii^ 



-. „ XVI. Early Reprksentations of Saturn 

From an illustration in the Systeina Satiirnium of Christian Huyghens. 



(Page 242) 



The Superior Planets 

called, from the Latin ansuy which means a handle. 
At length, in the year 1656, he solved the problem, 
and this he did by means of that 123-foot tubeless 
telescope, of which mention has already been made. 
The ring happened then to be at its edgewise period, 
and a careful study of the behaviour of the ansae 
when disappearing and reappearing soon revealed to 
Huyghens the true explanation. 



The Planets Uranus and Neptune 

We have already explained (in Chapter II.) the cir- 
cumstances in which both Uranus and Neptune were 
discovered. It should, however, be added that after 
the discovery of Uranus, that planet was found to 
have been already noted upon several occasions by 
different observers, but always without the least sus- 
picion that it was other than a mere faint star. Again, 
with reference to the discovery of Neptune, it may 
here be mentioned that the apparent amount by 
which that planet had pulled Uranus out of its place 
upon the starry background was exceedingly small — so 
small, indeed, that no eye could have detected it with- 
out the aid of a telescope ! 

Of the two predictions of the place of Neptune 
in the sky, that of Le Verrier was the nearer. Indeed, 
the position calculated by Adams was more than twice 
as far out. But Adams was by a long way the first 
in the field with his results, and only for unfortunate 
delays the prize would certainly have fallen to him. 
For instance, there was no star-map at Cambridge, 
and Professor Challis, the director of the observatory 
there, was in consequence obliged to make a laborious 

243 



Astronomy of To-day 

examination of the stars in the suspected region, On 
the other hand, all that Galle had to do was to 
compare that part of the sky where Le Verrier told 
him to look with the Berlin star-chart which he had 
by him. This he did on September 23, 1846, with 
the result that he quickly noted an eighth magnitude 
star which did not figure in that chart. By the next 
night this star had altered its position in the sky, 
thus disclosing the fact that it was really a planet. 

Six days later Professor Challis succeeded in finding 
the planet, but of course he was now too late. On 
reviewing his labours he ascertained that he had 
actually noted down its place early in August, and 
had he only been able to sift his observations as he 
made them, the discovery would have been made 
then. 

Later on it was found that Neptune had only just 
missed being discovered about fifty years earlier. In 
certain observations made during 1795, the famous 
French astronomer, Lalande, found that a star, which 
he had mapped in a certain position on the 8th of 
May of that year, was in a different position two days 
later. The idea of a planet does not appear to have 
entered his mind, and he merely treated the first 
observation as an error 1 

The reader will, no doubt, recollect how the dis- 
covery of the asteroids was due in effect to an 
apparent break in the seemingly regular sequence 
of the planetary orbits outwards from the sun. This 
curious sequence of relative distances is usually known 
as "Bode's Law," because it was first brought into 
general notice by an astronomer of that name. It 
had, however, previously been investigated mathemati- 

244 



The Superior Planets 

cally by Titius in 1772. Long before this, indeed, the 
unnecessarily wide space between the orbits of Mars 
and Jupiter had attracted the attention of the great 
Kepler to such a degree, that he predicted that a 
planet would some day be found to fill the void. 
Notwithstanding the service which the so-called Law 
of Bode has indirectly rendered to astronomy, it has 
strangely enough been found after all not to rest 
upon any scientific foundation. It will not account 
for the distance from the sun of the orbit of Neptune, 
and the very sequence seems on the whole to be in 
the nature of a mere coincidence. 

Neptune is invisible to the naked eye ; Uranus is 
just at the limit of visibility. Both planets are, how- 
ever, so far from us that we can get but the poorest 
knowledge of their condition and surroundings. 
Uranus, up to the present, is known to be attended 
by four satellites, and Neptune by one. The planets 
themselves are about equal in size ; their diameters, 
roughly speaking, being about one-half that of Saturn. 
Some markings have, indeed, been seen upon the disc 
of Uranus, but they are very indistinct and fleeting. 
From observation of them, it is assumed that the 
planet rotates on its axis in a period of some ten 
to twelve hours. No definite markings have as yet 
been seen upon Neptune, which body is described 
by several observers as resembling a faint planetary 
nebula. 

With regard to their physical condition, the most 
that can be said about these two planets is that they 
are probably in much the same vaporous state as 
Jupiter and Saturn. On account of their great dis- 
tance from the sun they can receive but little solar 

245 



Astronomy of To-day 

heat and light. Seen from Neptune, in fact, the sun 
would appear only about the size of Venus at her best, 
though of a brightness sufficiently intense to illumine 
the Neptunian landscape with about seven hundred 
times our full moonlight. 



246 



CHAPTER XIX 

COMETS 

The reader has, no doubt, been struck by the marked 
uniformity which exists among those members of the 
solar system with which we have dealt up to the 
present. The sun, the planets, and their satellites 
are all what we call solid bodies. The planets move 
around the sun, and the satellites around the planets, 
in orbits which, though strictly speaking, ellipses, are 
yet not in any instance of a very oval form. Two 
results naturally follow from these considerations. 
Firstly, the bodies in question hide the light coming 
to us from those further off, when they pass in front 
of them. Secondly, the planets never get so far from 
the sun that we lose sight of them altogether. 

With the objects known as Comets it is, however, 
quite the contrary. These objects do not conform 
to our notions of solidity. They are so transparent 
that they can pass across the smallest star without 
dimming its light in the slightest degree. Again, they 
are only visible to us during portion of their orbits. 
A comet may be briefly described as an illuminated 
filmy-looking object, made up usually of three por- 
tions — a head, a nucleus, or brighter central portion 
within this head, and a tail. The heads of comets 
vary greatly in size ; some, indeed, appear quite small, 
like stars, while others look even as large as the 

247 



Astronomy of To-day 

moon. Occasionally the nucleus is wanting, and 
sometimes the tail also. 

These mysterious visitors to our skies come up into 
view out of the immensities beyond, move towards 
the sun at a rapidly increasing speed, and, having 
gone around it, dash away again into the depths of 
space. As a comet approaches the sun, its body 



// 



Sun 



\ 



Fig. i8. — Showing how the Tail of a Comet is directed away from the Sun. 

appears to grow smaller and smaller, while, at the 
same time, it gradually throws out behind it an 
appendage like a tail. As the comet moves round the 
central orb this tail is always directed away from the 
sun ; and when it departs again into space the tail 
goes in advance. As the comet's distance from the 
sun increases, the tail gradually shrinks away and 
the head once more grows in size (see Fig. i8). In 
consequence of these changes, and of the fact that 
we lose sight of comets comparatively quickly, one 

248 



Comets 

is much inclined to wonder' what further changes 
may take place after the bodies have passed beyond 
our ken. 

The orbits of comets are, as we have seen, very 
elliptic. In some instances this ellipticity is so great 
as to take the bodies out into space to nearly six 
times the distance of Neptune from the sun. For a 
long time, indeed, it was considered that comets were 
of two kinds, namely, those which actually belonged 
to the solar system, and those which were merely 
visitors to it for the first and only time — rushing in 
from the depths of space, rapidly circuiting the sun, 
and finally dashing away into space again, never to 
return. On the contrary, nowadays, astronomers are 
generally inclined to regard comets as permanent 
members of the solar system. 

The difficulty, however, of deciding absolutely 
whether the orbits of comets are really always closed 
curves, that is to say, curves which must sooner or 
later bring the bodies back again towards the sun, 
is, indeed, very great. Comets, in the first place, are 
always so diffuse, that it is impossible to determine 
their exact position, or, rather, the exact position of 
that important point within them, known as the centre 
of gravity. Secondly, that stretch of its orbit along 
which we can follow a comet, is such a very small 
portion of the whole path, that the slightest errors 
of observation which we make will result in con- 
siderably altering our estimate of the actual shape of 
the orbit. 

Comets have been described as so transparent that 
they can pass across the sky without dimming the 
lustre of the smallest stars, which the thinnest fog 

249 



Astronomy of To-day 

or mist would do. This is, indeed, true of every 
portion of a comet except the nucleus, which is, as 
its name implies, the densest part. And yet, in con- 
trast to this ghostlike character, is the strange fact 
that when comets are of a certain brightness they 
may actually be seen in full daylight. 

As might be gathered from their extreme tenuity, 
comets are so exceedingly small in mass that they do 
not appear to exert any gravitational attraction upon 
the other bodies of our system. It is, indeed, a 
known fact that in the year 1886 a comet passed 
right amidst the satellites of Jupiter without disturbing 
them in the slightest degree. The attraction of the 
planet, on the other hand, so altered the comet's 
orbit, as to cause it to revolve around the sun in a 
period of seven years, instead of twenty-seven, as had 
previously been the case. Also, in 1779, the comet 
known as Lexell's passed quite close to Jupiter, and 
its orbit was so changed by that planet's attraction 
that it has never been seen since. The density of 
comets must, as a rule, be very much less than the 
one-thousandth part of that of the air at the surface 
of our globe ; for, if the density of the comet were 
even so small as this, its mass would not be in- 
appreciable. 

If comets are really undoubted members of the 
solar system, the circumstances in which they were 
evolved must have been different from those which 
produced the planets and satellites. The axial rota- 
tions of both the latter, and also their revolutions, take 
place in one certain direction ; ^ their orbits, too, are 

^ With the exception, of course, of such an anomaly as the retrograde 
motion of the ninth satellite of Saturn. 

250 



Comets 

ellipses which do not differ much from circles, and 
which, furthermore, are situated fairly in the one 
plane. Comets, on the other hand, do not necessarily 
travel round the sun in the same fixed direction as 
the planets. Their orbits, besides, are exceedingly 
elliptic ; and, far from keeping to one plane, or even 
near it, they approach the sun from all directions. 

Broadly speaking, comets may be divided into two 
distinct classes, or ^^ families." In the first class, the 
same orbit appears to be shared in common by a 
series of comets which travel along it, one following 
the other. The comets which appeared in the years 
1668, 1843, 1880, 1882, and 1887 are instances of a 
number of different bodies pursuing the same path 
around the sun. The members of a comet family 
of this kind are observed to have similar characteris- 
tics. The idea is that such comets are merely portions 
of one much larger cometary body, which became 
broken up by the gravitational action of other bodies 
in the system, or through violent encounter with the 
sun's surroundings. 

The second class is composed of comets which are 
supposed to have been seized by the gravitative action 
of certain planets, and thus forced to revolve in short 
ellipses around the sun, well within the limits of 
the solar system. These comets are, in consequence, 
spoken of as ^' captures." They move around the sun 
in the same direction as the planets do. Jupiter has a 
large comet family of this kind attached to him. As a 
result of his overpowering gravitation, it is imagined 
that during the ages he must have attracted a large 
number of these bodies on his own account, and, 
perhaps, have robbed other planets of their captures. 

251 



Astronomy of To-day 

His family at present numbers about thirty. Of the 
other planets, so far as we know, Saturn possesses a 
comet family of two, Uranus three, and Neptune six. 
There are, indeed, a few comets which appear as if 
under the influence of some force situated outside the 
known bounds of the solar system, a circumstance 
which goes to strengthen the idea that other planets 
may revolve beyond the orbit of Neptune. The 
terrestrial planets, on the other hand, cannot have 
comet families ; because the enormous gravitative 
action of the sun in their vicinity entirely overpowers 
the attractive force which they exert upon those 
comets which pass close to them. Besides this, a 
comet, when in the inner regions of the solar system, 
moves with such rapidity, that the gravitational pull 
of the planets there situated is not powerful enough 
to deflect it to any extent. It must not be presumed, 
however, that a comet once captured should always 
remain a prisoner. Further disturbing causes might 
unsettle its newly acquired orbit, and send it out 
again into the celestial spaces. 

With regard to the matter of which comets are 
composed, the spectroscope shows the presence in 
them of hydrocarbon compounds (a notable charac- 
teristic of these bodies), and at times, also, of sodium 
and iron. Some of the light which we get from 
comets is, however, merely reflected sunlight. 

The fact that the tails of comets are always directed 
away from the sun, has given rise to the idea that this 
is caused by some repelling action emanating from the 
sun itself, which is continually driving off the smallest 
particles. Two leading theories have been formu- 
lated to account for the tails themselves upon the 

252 



Comets 

above assumption. One of these, first suggested by 
Olbers in 1812, and now associated with the name of 
the Russian astronomer, the late Professor Br^dikhine, 
who carefully worked it out, presumes an electrical 
action emanating from the sun ; the other, that of 
Arrhenius, supposes a pressure exerted by the solar 
light in its radiation outwards into space. It is pos- 
sible, indeed, that repelling forces of both these kinds 
may be at work together. Minute particles are pro- 
bably being continually produced by friction and 
collisions among the more solid parts in the heads of 
comets. Supposing that such particles are driven off 
altogether, one may therefore assume that the so-called 
captured comets are disintegrating at a comparatively 
rapid rate. Kepler long ago maintained that " comets 
die," and this actually appears to be the case. The 
ordinary periodic ones, such, for instance, as Encke's 
Comet, are very faint, and becoming fainter at each 
return. Certain of these comets have, indeed, failed 
altogether to reappear. It is notable that the members 
of Jupiter's comet family are not very conspicuous 
objects. They have small tails, and even in some 
cases have none at all. The family, too, does not 
contain many members, and yet one cannot but 
suppose that Jupiter, on account of his great mass, 
has had many opportunities for making captures 
adown the ages. 

Of the two theories to which allusion has above 
been made, that of Br^dikhine has been worked out 
so carefully, and with such a show of plausibility, 
that it here calls for a detailed description. It ap- 
pears besides to explain the phenomena of comets' 
tails so much more satisfactorily than that of Arr- 

253 



Astronomy of To-day 

heniuS; that astronomers are inclined to accept it the 
more readily of the two. According to Bredikhine's 
theory the electrical repulsive force^ which he assumes 
for the purposes of his argument, will drive the 
minutest particles of the comet in a direction away 
from the sun much more readily than the gravitative 
action of that body will pull them towards it. This 
may be compared to the ease with which fine dust 
may be blown upwards, although the earth's gravi- 
tation is acting upon it all the time. 

The researches of Brddikhine, which began seriously 
with his investigation of Coggia's Comet of 1874, led 
him to classify the tails of comets in three types. 
Presuming that the repulsive force emanating from 
the sun did not vary, he came to the conclusion that 
the different forms assumed by cometary tails must 
be ascribed to the special action of this force upon 
the various elements which happen to be present in 
the comet. The tails which he classes as of the first 
type, are those which are long and straight and point 
directly away from the sun. Examples of such tails 
are found in the comets of 1811, 1843, and 1861. 
Tails of this kind, he thinks, are in all probability 
formed of hydrogen. His second type comprises 
those which are pointed away from the sun, but at 
the same time are considerably curved, as was seen 
in the comets of Donati and Coggia. These tails are 
formed of hydrocarbon gas. The*third type of tail is 
short, brush-like, and strongly bent, and is formed of 
the vapour of iron, mixed with that of sodium and 
other elements. It should, however, be noted that 
comets have occasionally been seen which possess 
several tails of these various types. 

254 



Comets 

We will now touch upon a few of the best known 
comets of modern times. 

The comet of 1680 was the first whose orbit was 
calculated according to the laws of gravitation. This 
was accomplished by Newton, and he found that 
the comet in question completed its journey round 
the sun in a period of about 600 years. 

In 1682 there appeared a great comet, which has 
become famous under the name of Halley's Comet, 
in consequence of the profound investigations made 
into its motion by the great astronomer, Edmund 
Halley. He fixed its period of revolution around the 
sun at about seventy-five years, and predicted that it 
would reappear in the early part of 1759. He did not, 
however, live to see this fulfilled, but the comet duly 
returned — the first body of the kind to verify such a 
prediction — and was detected on Christmas Day, 
1758, by George Palitzch, an amateur observer living 
near Dresden. Halley also investigated the past 
history of the comet, and traced it back to the year 
1456. The orbit of Halley's comet passes out slightly 
beyond the orbit of Neptune. At its last visit in 1835, 
this comet passed comparatively close to us, namely, 
within five million miles of the earth. According to 
the calculations of Messrs P. H. Cowell and A. C. D. 
Crommelin of Greenwich Observatory, its next return 
will be in the spring of 1910 ; the nearest approach to 
the earth taking place about May 12. 

On the 26th of March, 181 1, a great comet appeared, 
which remained visible for nearly a year and a half. 
It was a magnificent object ; the tail being about 
ICO millions of miles in length, and the head about 
127,000 miles in diameter. A detailed study which 

255 



Astronomy of To-day 

he gave to this comet prompted Gibers to put forward 
that theory of electrical repulsion which, as we have 
seen, has since been so carefully worked out by 
Br^dikhine. Gibers had noticed that the particles 
expelled from the head appeared to travel to the end 
of the tail in about eleven minutes, thus showing a 
velocity per second very similar to that of light. 

The discovery in 1819 of the comet known as 
Encke's, because its orbit was determined by an 
astronomer of that name, drew attention for the first 
time to Jupiter's comet family, and, indeed, to short- 
period comets in general. This comet revolves around 
the sun in the shortest known period of any of these 
bodies, namely, 3J years. Encke predicted that it 
would return in 1822. This duly occurred, the comet 
passing at its nearest to the sun within three hours 
of the time indicated ; being thus the second instance 
of the fulfilment of a prediction of the kind. A 
certain degree of irregularity which Encke's Comet 
displays in the dates of its returns to the sun, has 
been supposed to indicate that it passes in the course 
of its orbit through some retarding medium, but no 
definite conclusions have so far been arrived at in 
this matter. 

A comet, which appeared in 1826, goes by the 
name of Biela's Comet, because of its discovery by 
an Austrian military ojEficer, Wilhelm von Biela. 
This comet was found to have a period of between 
six and seven years. Certain calculations made by 
Gibers showed that, at its return in 1832, it would 
pass through the earth's orbit. The announcement 
of this gave rise to a panic ; for people did not wait 
to inquire whether the earth would be anywhere near 

256 




Plate XVII. Donati's Comet 

From a drawing made on October gtli, 1858. by G. P. Bond, of Harvard College Observatory, 

U.S.A. A good illustration of Br^dikiiine's theory: note the straight tails of his y/;.\V tyi)e. 

and tlie curved tail of his secotui. 

(ra-e .57) 



Comets 

that part of its orbit when the comet passed. The 
panic, however, subsided when the French astronomer, 
Arago, showed that at the moment in question the 
earth would be some 50 millions of miles away from 
the point indicated ! 

In 1846, shortly after one of its returns, Biela's 
Comet divided into two portions. At its next appear- 
ance (1852) these portions had separated to a distance 
of about i-J millions of miles from each other. This 
comet, or rather its constituents, have never since 
been seen. 

Perhaps the most remarkable comet of recent 
times was that of 1858, known as Donati's, it having 
been discovered at Florence by the Italian astronomer, 
G. B. Donati. This comet, a magnificent object, 
was visible for more than three months with the 
naked eye. Its tail was then 54 millions of miles 
in length. It was found to revolve around the sun 
in a period of over 2000 years, and to go out in its 
journey to about 5-J- times the distance of Neptune. 
Its motion is retrograde, that is to say, in the con- 
trary direction to the usual movement in the solar 
system. A number of beautiful drawings of Donati's 
Comet were made by the American astronomer, 
G. P. Bond. One of the best of these is reproduced 
on Plate XVII., p. 256. 

In 1861 there appeared a great comet. On the 
30th of June of that year the earth and moon actually 
passed through its tail ; but no effects were noticed, 
other than a peculiar luminosity in the sky. 

In the year 1881 there appeared another large 
comet, known as Tebbutt's Comet, from the name 
of its discoverer. This was iht first comet of which a 

257 R 



Astronomy of To-day 

satisfactory photograph was obtained. The photograph 
in question was taken by the late M. Janssen. 

The comet of 1882 was of vast size and brilliance. 
It approached so close to the sun that it passed 
through some 100,000 miles of the solar corona. 
Though its orbit was not found to have been altered 
by this experience, its nucleus displayed signs of 
breaking up. Some very fine photographs of this 
comet were obtained at the Cape of Good Hope by 
Mr. (now Sir David) Gill. 

The comet of 1889 was followed with the telescope 
nearly up to the orbit of Saturn, which seems to be 
the greatest distance at which a comet has ever been 
seen. 

^\it first discovery of a comet by photographic means^ 
was made by Professor Barnard in 1892 ; and, since 
then, photography has been employed with marked 
success in the detection of small periodic comets. 

The best comet seen in the Northern hemisphere 
since that of 1882, appears to have been Daniel's 
Comet of 1907 (see Plate XVIII., p. 258). This comet 
was discovered on June 9, 1907, by Mr. Z. Daniel, 
at Princeton Observatory, New Jersey, U.S.A. It 
became visible ^to the naked eye about mid-July of 
that year, and reached its greatest brilliancy about the 
end of August. It did not, however, attract much 
popular attention, as its position in the sky allowed 
it to be seen only just before dawn. 

^ If vfe except the case of the comet which was photographed near the 
solar corona in the eclipse of 1882. 



258 




Plate XVIII. Daniel's Comet of 1907 

From a photograph taken, on August nth, 1907, by Dr. Max Wolf, at the Astrophysical 
Observatory, Heidelberg. The instrument used was a 28-inch reflecting telescope, and the 
time of exposure was fifteen minutes. As the telescope was guided to follow the moving 
coniet, the stars have imprinted themselves upon the photographic plate as short trails". 
This is clearly the opposite to what is depicted on Plate XIH." (Page c^S) 



CHAPTER XX 

REMARKABLE COMETS 

If eclipses were a cause of terror in past ages, comets 
appear to have been doubly so. Their much longer 
continuance in the sight of men had no doubt some- 
thing to say to this, and also the fact that they arrived 
without warning ; it not being then possible to give 
even a rough prediction of their return, as in the case 
of eclipses. As both these phenomena were occa- 
sional, and out of the ordinary course of things, they 
drew exceptional attention as unusual events always 
do ; for it must be allowed that quite as wonderful 
things exist, but they pass unnoticed merely because 
men have grown accustomed to them. 

For some reason the ancients elected to class 
comets along with meteors, the aurora borealis, 
and other phenomena of the atmosphere, rather 
than with the planets and the bodies of the spaces 
beyond. The sudden appearance of these objects 
led them to be regarded as signs sent by the gods 
to announce remarkable events, chief among these 
being the deaths of monarchs. Shakespeare has 
reminded us of this in those celebrated lines in 
Julius Ccesar : — 

" When beggars die there are no comets seen, 
The heavens themselves blaze forth the death of princes." 

Numbed by fear, the men of old blindly accepted 
these presages of fate ; and did not too closely 

259 



Astronomy of To-day 

question whether the threatened danger was to 
their own nation or to some other, to their ruler 
or to his enemy. Now and then, as in the case of 
the Roman Emperor Vespasian, there was a cynical 
attempt to apply some reasoning to the portent. 
That emperor, in alluding to the comet of A.D. 79, 
is reported to have said : ''This hairy star does not 
concern me ; it menaces rather the King of the 
Parthians, for he is hairy and I am bald." Vespasian, 
all the same, died shortly afterwards ! 

Pliny, in his natural history, gives several instances 
of the terrible significance which the ancients attached 
to comets. ''A comet," he says, ''is ordinarily a very 
fearful star ; it announces no small effusion of blood. 
We have seen an example of this during the civil 
commotion of Octavius." 

A very brilliant comet appeared in 371 B.C., and 
about the same time an earthquake caused Helice 
and Bura, two towns in Achaia, to be swallowed up 
by the sea. The following remark made by Seneca 
concerning it shows that the ancients did not con- 
sider comets merely as precursors, but even as actual 
causes of fatal events : " This comet, so anxiously 
observed by every one, because of the great catastrophe 
which it produced as soon as it appeared, the submersion 
of Bura and Helice." 

Comets are by no means rare visitors to our skies, 
and very few years have elapsed in historical times 
without such objects making their appearance. In 
the Dark and Middle Ages, when Europe was split up 
into many small kingdoms and principalities, it was, 
of course, hardly possible for a comet to appear with- 
out the death of some ruler occurring near the time. 

260 



Remarkable Comets 

Critical situations, too, were continually arising in 
those disturbed days. The end of Louis le Debon- 
naire was hastened, as the reader will, no doubt, 
recollect, by the great eclipse of 840 ; but it was 
firmly believed that a comet which had appeared 
a year or two previously presaged his death. The 
comet of 1556 is reported to have influenced the ab- 
dication of the Emperor Charles V. ; but curiously 
enough, this event had already taken place before the 
comet made its appearance ! Such beliefs, no doubt, 
had a very real effect upon rulers of a superstitious 
nature, or in a weak state of health. For instance, 
Gian Galeazzo Visconti, Duke of Milan, was sick 
when the comet of 1402 appeared. After seeing it, 
he is said to have exclaimed : " I render thanks to 
God for having decreed that my death should be 
announced to men by this celestial sign." His malady 
then became worse, and he died shortly afterwards. 

It is indeed not improbable that such superstitious 
fears in monarchs were fanned by those who would 
profit by their deaths, and yet did not wish to stain 
their own hands with blood. 

Evil though its effects may have been, this morbid 
interest which past ages took in comets has proved of 
the greatest service to our science. Had it not been 
believed that the appearance of these objects was 
attended with far-reaching effects, it is very doubtful 
whether the old chroniclers would have given them- 
selves the trouble of alluding to them at all ; and thus 
the modern investigators of cometary orbits would 
have lacked a great deal of important material. 

We will now mention a few of the most notable 
comets which historians have recorded. 

261 



Astronomy of To-day 

A comet which appeared in 344 B.C. was thought 
to betoken the success of the expedition undertaken 
in that year by Timoleon of Corinth against Sicily. 
"The gods by an extraordinary prodigy announced 
his success and future greatness : a burning torch 
appeared in the heavens throughout the night and 
preceded the fleet of Timoleon until it arrived off the 
coast of Sicily." 

The comet of 43 B.C. was generally believed to be 
the soul of Caesar on its way to heaven. 

Josephus tells us that in A.D. 69 several prodigies, 
and amongst them a comet in the shape of a sword, 
announced the destruction of Jerusalem. This comet 
is said to have remained over the city for the space of 
a year 1 

A comet which appeared in a.d. 336 was considered 
to have announced the death of the Emperor Con- 
stantine. 

But perhaps the most celebrated comet of early 
times was the one which appeared in a.d, 1000. That 
year was, in more than one way, big with portent, 
for there had long been a firm belief that the Chris- 
tian era could not possibly run into four figures. Men, 
indeed, steadfastly believed that when the thousand 
years had ended, the millennium would immediately 
begin. Therefore they did not reap neither did they 
sow, they toiled not, neither did they spin, and the 
appearance of the comet strengthened their convic- 
tions. The fateful year, however, passed by without 
anything remarkable taking place ; but the neglect of 
husbandry brought great famine and pestilence over 
Europe in the years which followed. 

In April 1066, that year fraught with such immense 
262 



Remarkable Comets 



consequences for England, a comet appeared. No 
one doubted but that it was a presage of the success 
of the Conquest; and perhaps, indeed, it had its due 
weight in determining the minds and actions of the 
men who took part in the expedition. Nova stelluy 
novus rex ('^a new star, a new sovereign") was a 



^ISTIMIRANT/^SmC^ 





Fig. 19. — The comet of 1066, as represented in the Bayeux Tapestry. 
(From the World of Comets.) 

favourite proverb of the time. The chroniclers, with 
one accord, have delighted to relate that the Normans, 
<' guided by a comet," invaded England. A represen- 
tation of this object appears in the Bayeux Tapestry 
(see Fig. 19, p. 263).! 

^ With regard to the words " Isti mirant Stella" in the figure, Mr. 
W. T. Lynn suggests that they may not, after all, be the grammatically 
bad Latin which they appear, but that the legend is really *' Isti mirantur 
stellam," the missing letters being supposed to be hidden by the building 
and the comet. 

263 



Astronomy of To-day 

We have mentioned Halley's Comet of 1682, and 
how it revisits the neighbourhood of the earth at 
intervals of seventy-six years. The comet of 1066 has 
for many years been supposed to be Halley's Comet 
on one of its visits. The identity of these two, 
however, was only quite recently placed beyond all 
doubt by the investigations of Messrs Cowell and 
Crommelin. This comet appeared also in 1456, when 
John Huniades was defending Belgrade against the 
Turks led by Mahomet II., the conqueror of Con- 
stantinople, and is said to have paralysed both armies 
with fear. 

The Middle Ages have left us descriptions of comets, 
which show only too well how the imagination will 
run riot under the stimulus of terror. For instance, 
the historian, Nicetas, thus describes the comet of the 
year 1182 : ^' After the Romans were driven from 
Constantinople a prognostic was seen of the excesses 
and crimes to which Andronicus was to abandon him- 
self. A comet appeared in the heavens similar to a 
writhing serpent ; sometimes it extended itself, some- 
times it drew itself in ; sometimes, to the great terror 
of the spectators, it opened a huge mouth ; it seemed 
that, as if thirsting for human blood, it was upon the 
point of satiating itself." And, again, the celebrated 
Ambrose Pare, the father of surgery, has left us the 
following account of the comet of 1528, which ap- 
peared in his own time : " This comet," said he, ^^ was 
so horrible, so frightful, and it produced such great 
terror in the vulgar, that some died of fear, and others 
fell sick. It appeared to be of excessive length, and 
was of the colour of blood. At the summit of it was 
seen the figure of a bent arm, holding in its hand a 

264 



Remarkable Comets 

great sword, as if about to strike. At the end of the 
point there were three stars. On both sides of the 
rays of this comet were seen a great number of axes, 
knives, blood-coloured swords, among which were a 
great number of hideous human faces, with beards and 
bristling hair." Par6, it is true, was no astronomer ; 
yet this shows the effect of the phenomenon, even 
upon a man of great learning, as undoubtedly he was. 
It should here be mentioned that nothing very re- 
markable happened at or near the year 1528. 

Concerning the comet of 1680, the extraordinary 
story got about that, at Rome, a hen had laid an 
egg on which appeared a representation of the 
comet ! 

But the superstitions with regard to comets were 
now nearing their end. The last blow was given by 
Halley, who definitely proved that they obeyed the 
laws of gravitation, and circulated around the sun as 
planets do ; and further announced that the comet 
of 1682 had a period of seventy-six years, which 
would cause it to reappear in the year 1759. We 
have seen how this prediction was duly verified. We 
have seen, too, how this comet appeared again in 1835, 
and how it is due to return in the early part of 1910. 



265 



CHAPTER XXI 

METEORS OR SHOOTING STARS 

Any one who happens to gaze at the sky for a 
short time on a clear night is pretty certain to be 
rewarded with a view of what is popularly known 
as a ^^ shooting star." Such an object, however, is 
not a star at all, but has received its appellation 
from an analogy ; for the phenomenon gives to the 
inexperienced in these matters an impression as if 
one of the many points of light, which glitter in the 
vaulted heaven, had suddenly become loosened from 
its place, and was falling towards the earth. In its 
passage across the sky the moving object leaves 
behind a trail of light which usually lasts for a few 
moments. Shooting stars, or meteors, as they are 
technically termed, are for the most part very small 
bodies, perhaps no larger than peas or pebbles, 
which, dashing towards our earth from space be- 
yond, are heated to a white heat, and reduced to 
powder by the friction resulting from their rapid 
passage into our atmosphere. This they enter at 
various degrees of speed, in some cases so great as 
45 miles a second. The speed, of course, will depend 
greatly upon whether the earth and the meteors are 
rushing towards each other, or whether the latter 
are merely overtaking the earth. In the first of 
these cases the meteors will naturally collide with 

266 



Meteors or Shooting Stars 

the atmosphere with great force ; in the other case 
they will plainly come into it with much less rapidity. 
As has been already stated, it is from observations 
of such bodies that we are enabled to estimate, 
though very imperfectly, the height at which the 
air around our globe practically ceases, and this 
height is imagined to be somewhere about loo miles. 
Fortunate, indeed, is it for us that there is a goodly 
layer of atmosphere over our heads, for, were this 
not so, these visitors from space would strike upon 
the surface of our earth night and day, and render 
existence still more unendurable than many persons 
choose to consider it. To what a bombardment 
must the moon be continually subject, destitute as 
she is of such an atmospheric shield 1 

It is only in the moment of their dissolution that 
we really learn anything about meteors, for these 
bodies are much too small to be seen before they 
enter our atmosphere. The debris arising from their 
destruction is wafted over the earth, and, settling 
down eventually upon its surface, goes to augment 
the accumulation of that humble domestic commodity 
which men call dust. This continual addition of 
material tends, of course, to increase the mass of the 
earth, though the effect thus produced will be on 
an exceedingly small scale. 

The total number of meteors moving about in 
space must be practically countless. The number 
which actually dash into the earth's atmosphere 
during each year is, indeed, very great. Professor 
Simon Newcomb, the well-known American astrono- 
mer, has estimated that, of the latter, those large 
enough to be seen with the naked eye cannot be in 

267 



Astronomy of To-day 

all less than i46;OoO;000,ooo per annum. Ten times 
more numerous still are thought to be those insig- 
nificant ones which are seen to pass like mere 
sparks of light across the field of an observer's 
telescope. 

Until comparatively recent times, perhaps up to 
about a hundred years ago, it was thought that 
meteors were purely terrestrial phenomena which 
had their origin in the upper regions of the air. It, 
however, began to be noticed that at certain periods 
of the year these moving objects appeared to come 
from definite areas of the sky. Considerations, there- 
fore, respecting their observed velocities, directions, 
and altitudes, gave rise to the theory that they are 
swarms of small bodies travelling around the sun in 
elongated elliptical orbits, all along the length of 
which they are scattered, and that the earth, in its 
annual revolution, rushing through the midst of such 
swarms at the same epoch each year, naturally en- 
tangles many of them in its atmospheric net. 

The dates at which the earth is expected to pass 
through the principal meteor-swarms are now pretty 
well known. These swarms are distinguished from 
one another by the direction of the sky from which 
the meteors seem to arrive. Many of the swarmxS 
are so wide that the earth takes days, and even 
weeks, to pass through them. In some of these 
swarms, or streams, as they are also called, the 
meteors are distributed with fair evenness along 
the entire length of their orbits, so that the earth is 
greeted with a somewhat similar shower at each 
yearly encounter. In others, the chief portions are 
bunched together, so that, in certain years, the display 

268 



Meteors or Shooting Stars 

is exceptional (see Fig. 20, p. 269). That part of the 
heavens from which a shower of meteors is seen to 
emanate is called the ^'radiant/' or radiant point, 
because the foreshortened view we get of the streaks 
of light makes it appear as if they radiated outwards 
from this point. In observations of these bodies the 
attention of astronomers is directed to registering the 

S^arm or Meteors , . 



Far/A. 




Fig. 20. — Passage of the Earth through the thickest portion of a Meteor 
Swarm. The Earth and the Meteors are here represented as approach- 
ing each other from opposite directions. 

path and speed of each meteor, and to ascertaining 
the position of the radiant. It is from data such as 
these that computations concerning the swarms and 
their orbits are made. 

For the present state of knowledge concerning 
meteors, astronomy is largely indebted to the re- 
searches of Mr. W. F. Denning, of Bristol, and of the 
late Professor A. S. Herschel, 

During the course of each year the earth encounters 
a goodly number of meteor-swarms. Three of these, 

269 



Astronomy of To-day 

giving rise to fine displays, are very well known — the 
^' Perseids/' or August Meteors, and the ^* Leonids " 
and ^^ Bielids," which appear in November. 

Of the above three the Leonid display is by far the 
most important, and the high degree of attention paid 
to it has laid the foundation of meteoric astronomy 
in much the same way that the study of the fascinat- 
ing corona has given such an impetus to our know- 
ledge of the sun. The history of this shower of 
meteors may be traced back as far as A.D. 902, which 
was known as the " Year of the Stars." It is related 
that in that year, on the night of October 12th — the 
shower now comes about a month later — whilst the 
Moorish King, Ibrahim Ben Ahmed, lay dying before 
Cosenza, in Calabria, ^^a multitude of falling stars 
scattered themselves across the sky like rain," and the 
beholders shuddered at what they considered a dread 
celestial portent. We have, however, little knowledge 
of the subsequent history of the Leonids until 1698, 
since which time the maximum shower has appeared 
with considerable regularity at intervals of about thirty- 
three years. But it was not until 1799 that they 
sprang into especial notice. On the nth November 
in that year a splendid display was witnessed at 
Cumana, in South America, by the celebrated travellers, 
Humboldt and Bonpland. Finer still, and surpassing 
all displays of the kind ever seen, was that of Novem- 
ber 12, 1833, when the meteors fell thick as snowflakes, 
240,000 being estimated to have appeared during seven 
hours. Some of them were even so bright as to be 
seen in full daylight. The radiant from which the 
meteors seem to diverge was ascertained to be situated 
in the head of the constellation of the Lion, or ^' Sickle 

270 



Meteors or Shooting Stars 

of Leo/' as it is popularly termed; whence their name 
— Leonids. It was from a discussion of the observa- 
tions then made that the American astronomer, 
Olmsted; concluded that these meteors sprang upon 
us from interplanetary space, and were not, as had 
been hitherto thought, born of our atmosphere. Later 
on, in 1837, Olbers formulated the theory that the 
bodies in question travelled around the sun in an 
elliptical orbit, and at the same time he established 
the periodicity of the maximum shower. 

The periodic time of recurrence of this maximum, 
namely, about thirty-three years, led to eager expec- 
tancy as 1866 drew near. Hopes were then fulfilled, 
and another splendid display took place, of which Sir 
Robert Ball, who observed it, has given a graphic 
description in his Story of the Heavens. The dis- 
play was repeated upon a smaller scale in the two 
following years. The Leonids were henceforth deemed 
to hold an anomalous position among meteor swarms. 
According to theory the earth cut through their orbit 
at about the same date each year, and so a certain 
number were then seen to issue from the radiant. 
But, in addition, after intervals of thirty-three years, 
as has been seen, an exceptional display always took 
place ; and this state of things was not limited to one 
year alone, but was repeated at each meeting for about 
three years running. The further assumption was, 
therefore, made that the swarm was much denser in 
one portion of the orbit than elsewhere,^ and that this 
congested part was drawn out to such an extent that 
the earth could pass through the crossing place during 

^ The '* gem " of the meteor ring, as it has been termed. 
271 



Astronomy of To-day 

several annual meetings, and still find it going by like 
a long procession (see Fig. 20; p. 269). 

In accordance with this ascertained period of thirty- 
three years, the recurrence of the great Leonid shower 
was timed to take place on the 15th of November 1899. 
But there was disappointment then, and the displays 
which occurred during the few years following were not 
of much importance. A good deal of comment was 
made at the time, and theories were accordingly put 
forward to account for the failure of the great shower. 
The most probable explanation seems to be, that the 
attraction of one of the larger planets — Jupiter per- 
haps — has diverted the orbit somewhat from its old 
position, and the earth does not in consequence cut 
through the swarm in the same manner as it used 
to do. 

The other November display alluded to takes place 
between the 23rd and 27th of that month. It is called 
the Andromedid Shower, because the meteors appear 
to issue from the direction of the constellation of 
Andromeda, which at that period of the year is well 
overhead during the early hours of the night. These 
meteors are also known by the name of Btelids, from 
a connection which the orbit assigned to them appears 
to have with that of the well-known comet of Biela. 

M. Egenitis, Director of the Observatory of Athens, 
accords to the Bielids a high antiquity. He traces 
the shower back to the days of the Emperor Justinian. 
Theophanes, the Chronicler of that epoch, writing of 
the famous revolt of Nika in the year a.d. 532, says : — 
" During the same year a great fall of stars came from 
the evening till the dawn." M. Egenitis notes another 
early reference to these meteors in A.D. 752, during 

272 



Meteors or Shooting Stars 

the reign of the Eastern Emperor, Constantine 
Copronymous. Writing of that year, Nicephorus, a 
Patriarch of Constantinople, has as follows : — '^ All 
the stars appeared to be detached from the sky, and 
to fall upon the earth." 

The Bielids, however, do not seem to have attracted 
particular notice until the nineteenth century. Atten- 
tion first began to be riveted upon them on account 
of their suspected connection with Biela's comet. It 
appeared that the same orbit was shared both by that 
comet and the Bielid swarm. It will be remembered 
that the comet in question was not seen after its 
appearance in 1852. Since that date, however, the 
Bielid shower has shown an increased activity; which 
was further noticed to be especially great in those 
years in which the comet, had it still existed, would 
be due to pass near the earth. 

The third of these great showers to which allusion 
has above been made, namely, the Persezds, strikes the 
earth about the loth of August ; for which reason it 
is known on the Continent under the name of the 
"tears of St. Lawrence," the day in question being 
sacred to that Saint. This shower is traceable back 
many centuries, even as far as the year a.d. 811. 
The name given to these meteors, " Perseids," arises 
from the fact that their radiant point is situated in 
the constellation of Perseus. This shower is, however, 
not by any means limited to the particular night of 
August loth, for meteors belonging to the swarm may 
be observed to fall in more or less varying quantities 
from about July 8th to August 22nd. The Perseid 
meteors sometimes fall at the rate of about sixty per 
hour. They are noted for their great rapidity of 

273 s 



Astronomy of To-day 

motion, and their trails besides often persist for a 
minute or two before being disseminated. Unlike the 
other well-known showers, the radiants of which are 
stationary, that of the Perseids shifts each night a 
little in an easterly direction. 

The orbit of the Perseids cuts that of the earth 
almost perpendicularly. The bodies are generally 
supposed to be the result of the disintegration of an 
ancient comet which travelled in the same orbit. 
Tuttle's Comet, which passed close to the earth in 
1862, also belongs to this orbit ; and its period of 
revolution is calculated to be 131 years. The Perseids 
appear to be disseminated all along this great orbit, 
for we meet them in considerable quantities each year. 
The bodies in question are in general particularly 
small. The swarm has, however, like most others, 
a somewhat denser portion, and through this the 
earth passed in 1848. The aphelion, or point where 
the far end of the orbit turns back again towards 
the sun, is situated right away beyond the path of Nep- 
tune, at a distance of forty-eight times that of the earth 
from the sun. The comet of 1532 also belongs to the 
Perseid orbit. It revisited the neighbourhood of the 
earth in 1661, and should have returned in 1789, But 
we have no record of it in that year ; for which 
omission the then politically disturbed state of Europe 
may account. If not already disintegrated, this comet 
is due to return in 1919. 

This supposed connection between comets and 
meteor-swarms must be also extended to the case of 
the Leonids. These meteors appear to travel along 
the same track as Tempel's Comet of 1866. 

It is considered that the attractions of the various 
274 



Meteors or Shooting Stars 

bodies of the solar system upon a meteor swarm must 
eventually result in breaking up the ^'bunched" 
portion, so that in time the individual meteors should 
become distributed along the whole length of the 
orbit. Upon this assumption the Perseid swarm, in 
which the meteors are fairly well scattered along its 
path, should be of greater age than the Leonid. As 
to the Leonid swarm itself, Le Verrier held that it 
was first brought into the solar system in A.D. 126, 
having been captured from outer space by the gravi- 
tative action of the planet Uranus. 

The acknowledged theory of meteor swarms has 
naturally given rise to an idea, that the sunlight shin- 
ing upon such a large collection of particles ought 
to render a swarm visible before its collision with the 
earth's atmosphere. Several attempts have therefore 
been made to search for approaching swarms by 
photography, but, so far, it appears without success. 
It has also been proposed, by Mr. W. H. S. Monck, 
that the stars in those regions from which swarms 
are due, should be carefully watched, to see if their 
light exhibits such temporary diminutions as would 
be likely to arise from the momentary interposition 
of a cloud of moving particles. 

Between ten and fifteen years ago it happened that 
several well-known observers, employed in telescopic 
examination of the sun and moon, reported that from 
time to time they had seen small dark bodies, some- 
times singly, sometimes in numbers, in passage across 
the discs of the luminaries. It was concluded that 
these were meteors moving in space beyond the at- 
mosphere of the earth. The bodies were called ^^ dark 
meteors," to emphasise the fact that they were seen 

275 



Astronomy of To-day 

in their natural condition, and not in that momentary 
one in which they had hitherto been always seen ; i.e. 
when heated to white heat, and rapidly vaporised, in 
the course of their passage through the upper regions 
of our air. This ^^ discovery" gave promise of such 
assistance to meteor theories, that calculations were 
made from the directions in which they had been 
seen to travel, and the speeds at which they had 
moved, in the hope that some information concerning 
their orbits might be revealed. But after a while 
some doubt began to be thrown upon their being 
really meteors, and eventually an Australian observer 
solved the mystery. He found that they were merely 
tiny particles of dust, or of the black coating on the 
inner part of the tube of the telescope, becoming 
detached from the sides of the eyepiece and falling 
across the field of view. He was led to this con- 
clusion by having noted that a gentle tapping of his 
instrument produced the ^^ dark " bodies in great 
numbers I Thus the opportunity of observing meteors 
beyond our atmosphere had once miore failed. 

Meteorites, also known as aerolites and fireballs, 
are usually placed in quite a separate category from 
meteors. They greatly exceed the latter in size, are 
comparatively rare, and do not appear in any way 
connected with the various showers of meteors. The 
friction of their passage through the atmosphere 
causes them to shine with a great light ; and if not 
shattered to pieces by internal explosions, they reach 
the ground to bury themselves deep in it w^ith a 
great rushing and noise. When found by uncivilised 
peoples, or savages, they are, on account of their 
celestial origin, usually regarded as objects of wonder 

276 



Meteors or Shooting Stars 

and of worship, and thus have arisen many mytho- 
logical legends and deifications of blackened stones. 
On the other hand, when they get into the possession 
of the civilised, they are subjected to careful examina- 
tions and tests in chemical laboratories. The bodies 
are, as a rule, composed of stone, in conjunction with 
iron, nickel, and such elements as exist in abundance 
upon our earth ; though occasionally specimens are 
found which are practically pure metal. In the 
museums of the great capitals of both Continents are 
to be seen some fine collections of meteorites. Several 
countries — Greenland and Mexico, for instance — con- 
tain in the soil much meteoric iron, often in masses 
so large as to bafHe all attempts at removal. Blocks 
of this kind have been known to furnish the natives 
in their vicinity for many years with sources of work- 
able iron. 

The largest meteorite in the world is one known 
as the Anighito meteorite. It was brought to the 
United States by the explorer Peary, who found it at 
Cape York in Greenland. He estimates its weight at 
from 90 to 100 tons. One found in Mexico, called the 
Bacubirito, comes next, with an estimated weight of 
27J tons. The third in size is the Willamette meteo- 
rite, found at Willamette in Oregon in 1902. It 
measures 10X6JX4J feet, and weighs about 15J tons. 



277 



CHAPTER XXII 

THE STARS 

In the foregoing chapters we have dealt at length 
with those celestial bodies whose nearness to us brings 
them into our especial notice. The entire room, how- 
ever, taken up by these bodies, is as a mere point in 
the immensities of star-filled space. The sun, too, is 
but an ordinary star ; perhaps quite an insignificant 
one^ in comparison with the majority of those which 
stud that background of sky against which the planets 
are seen to perform their wandering courses. 

Dropping our earth and the solar system behind, 
let us go afield and explore the depths of space. 

We have seen how, in very early times, men por- 
tioned out the great mass of the so-called ^' fixed 
stars " into divisions known as constellations. The 
various arrangements, into which the brilliant points 
of light fell as a result of perspective, were noticed 
and roughly compared with such forms as were 
familiar to men upon the earth. Imagination quickly 
saw in them the semblances of heroes and of mighty 
fabled beasts ; and, around these monstrous shapes, 
legends were woven, which told how the great deeds 
done in the misty dawn of historical time had been 

^ Vega, for instance, shines one hundred times more brightly than the 
sun would do, were it to be removed to the distance at which that star is 
from us. 

278 



The Stars 

enshrined by the gods in the sky as an example and 
a memorial for men. Though the centuries have 
long outlived such fantasies, yet the constellation 
figures and their ancient names have been retained 
to this day, pretty well unaltered for want of any 
better arrangement. The Great and Little Bears, 
Cassiopeia, Perseus, and Andromeda, Orion and the 
rest, glitter in our night skies just as they did centuries 
and centuries ago. 

Many persons seem to despair of gaining any real 
knowledge of astronomy, merely because they are 
not versed in recognising the constellations. For 
instance, they will say : — ^^ What is the use of my 
reading anything about the subject ? Why, I believe 
I couldn't even point out the Great Bear, were I 
asked to do so ! " But if such persons will only con- 
sider for a moment that what we call the Great Bear 
has no existence in fact, they need not be at all dis- 
heartened. Could we but view this familiar constella- 
tion from a different position in space, we should 
perhaps be quite unable to recognise it. Mountain 
masses, for instance, when seen from new directions, 
are often unrecognisable. 

It took, as we have seen, a very long time for men 
to acknowledge the immense distances of the stars 
from our earth. Their seeming unchangeableness of 
position was, as we have seen, largely responsible for 
the idea that the earth was immovable in space. It 
is a wonder that the Copernican system ever gained 
the day in the face of this apparent fixity of the stars. 
As time went on, it became indeed necessary to accord 
to these objects an almost inconceivable distance, 
in order to account for the fact that they remained 

279 



Astronomy of To-day 

apparently quite undisplaced, notwithstanding the 
journey of millions of miles which the earth was 
now acknowledged to make each year around the 
sun. In the face of the gradual and immense improve- 
ment in telescopes, this apparent immobility of the 
stars was, however, not destined to last. The first 
ascertained displacement of a star, namely that of 
6i Cygni, noted by Bessel in the year 1838, definitely 
proved to men the truth of the Copernican system. 
Since then some forty more stars have been found to 
show similar tiny displacements. We are, therefore, 
in possession of the fact, that the actual distances of 
a few out of the great host can be calculated. 

To mention some of these. The nearest star to the 
earth, so far as we yet know, is Alpha Centauri, which 
is distant from us about 25 billions of miles. The 
light from this star, travelling at the stupendous rate 
of about 186,000 miles per second, takes about 4J 
years to reach our earth, or, to speak astronomically. 
Alpha Centauri is about 4.^ '^ light years " distant from 
us. Sirius — the brightest star in the whole sky — is at 
twice this distance, i.e. about 8|- light years. Vega is 
about 30 light years distant from us, Capella about 32, 
and Arcturus about 100. 

The displacements, consequent on the earth's move- 
ment, have, however, plainly nothing to say to any 
real movements on the part of the stars themselves. 
The old idea was that the stars were absolutely fixed ; 
hence arose the term " fixed stars " — a term which, 
though inaccurate, has not yet been entirely banished 
from the astronomical vocabulary. But careful obser- 
vations extending over a number of years have shown 
slight changes of position among these bodies ; and 

280 



The Stars 

such alterations cannot be ascribed to the revolution 
of the earth in its orbit, for they appear to take place 
in every direction. These evidences of movement 
are known as '^ proper motions/' that is to say, actual 
motions in space proper to the stars themselves. 
Stars which are comparatively near to us show, as a 
rule, greater proper motions than those which are 
farther off. It must not, however, be concluded that 
these proper motions are of any very noticeable 
amounts. They are, as a matter of fact, merely upon 
the same apparently minute scale as other changes 
in the heavens ; and would largely remain unnoticed 
were it not for the great precision of modern astro- 
nomical instruments. 

One of the swiftest moving of the stars is a star of 
the sixth magnitude in the constellation of the Great 
Bear ; which is known as ^' 1830 Groombridge," be- 
cause this was the number assigned to it in a catalogue 
of stars made by an astronomer of that name. It is 
popularly known as the '^Runaway Star," a name 
given to it by Professor Newcomb. Its speed is 
estimated to be at least 138 miles per second. It may 
be actually moving at a much greater rate, for it is 
possible that we see its path somewhat foreshortened. 

A still greater proper motion — the greatest, in fact, 
known — is that of an eighth magnitude star in the 
southern hemisphere, in the constellation of Pictor. 
Nothing, indeed, better shows the enormous distance 
of the stars from us, and the consequent inability of 
even such rapid movements to alter the appearance of 
the sky during the course of ages, than the fact that 
it would take more than two centuries for the star in 
question to change its position in the sky by a space 

281 



Astronomy of To-day 

equal to the apparent diameter of the moon ; a state- 
ment which is equivalent to saying that, were it 
possible to see this star with the naked eye, which it 
is not, at least twenty-five years would have to elapse 
before one would notice that it had changed its place 
at all ! 

Both the stars just mentioned are very faint. That 
in Pictor is, as has been said, not visible to the naked 
eye. It appears besides to be a very small body, for 
Sir David Gill finds a parallax which makes it only as 
far off from us as Sirius. The Groombridge star, too, 
is just about the limit of ordinary visibility. It is, in- 
deed, a curious fact that the fainter stars seem, on the 
average, to be moving more rapidly than the brighter. 

Investigations into proper motions lead us to think 
that every one of the stars must be moving in space 
in some particular direction. To take a few of the 
best known. Sirius and Vega are both approaching 
our system at a rate of about lo miles per second, 
Arcturus at about 5 miles per second, while Capella 
is receding from us at about 15 miles per second. 
Of the twin brethren. Castor and Pollux, Castor is 
moving away from us at about 4J miles per second, 
while Pollux is coming towards us at about 33 miles 
per second. 

Much of our knowledge of proper motions has been 
obtained indirectly by means of the spectroscope, 
on the Doppler principle already treated of, by which 
we are enabled to ascertain whether a source from 
which light is coming is approaching or receding. 

The sun being, after all, a mere star, it will appear 
only natural for it also to have a proper motion of 
its own. This is indeed the case ; and it is rushing 

282 



The Stars 

along in space at a rate of between ten and twelve 
miles per second; carrying with it its whole family 
of planets and satellites, of comets and meteors. 
The direction in which it is advancing is towards 
a point in the constellation of Lyra, not far from 
its chief star Vega. This is shown by the fact that 
the stars about the region in question appear to be 
opening out slightly, while those in the contrary 
portion of the sky appear similarly to be closi ^g 
together. 

Sir William Herschel was the first to discover this 
motion of the sun through space ; though in the 
idea that such a movement might take place he seems 
to have been anticipated by Mayer in 1760, by Michell 
in 1767, and by Lalande in 1776. 

A suggestion has been made that our solar system, 
in its motion through the celestial spaces, may 
occasionally pass through regions where abnormal 
magnetic conditions prevail, in consequence of which 
disturbances may manifest themselves throughout 
the system at the same instant. Thus the sun may 
be getting the credit of producing what it merely 
reacts to in common with the rest of its family. 
But this suggestion, plausible though it may seem, 
will not explain why the magnetic disturbances ex- 
perienced upon our earth show a certain dependence 
upon such purely local facts, as the period of the sun's 
rotation, for instance. 

One would very much like to know whether the 
movement of the sun is along a straight line, or in 
an enormous orbit around some centre. The idea 
has been put forward that it may be moving around 
the centre of gravity of the whole visible stellar 

283 



Astronomy of To-day 

universe. Madler, indeed, propounded the notion that 
Alcyone — the chief star in the group known as the 
Pleiades — occupied this centre, and that everything 
revolved around it. He went even further to pro- 
claim that here w^as the Place of the Almighty, the 
Mansion of the Eternal ! But Madler's ideas upon 
this point have long been shelved. 

To return to the general question of the proper 
niotion of stars. 

In several instances these motions appear to take 
place in groups, as if certain stars were in some -way 
associated together. For example, a large number 
of the stars composing the Pleiades appear to be 
moving through space in the same direction. Also, 
of the seven stars composing the Plough, all but 
two — the star at the end of its ^'handle," and that 
one of the " pointers," as they are called, which is 
the nearer to the pole star — have a common proper 
motion, Le. are moving in the same direction and 
nearly at the same rate. 

Further still, the well-known Dutch astronomer, 
Professor Kapteyn, of Groningen, has lately reached 
the astonishing conclusion that a great part of the 
visible universe is occupied by two vast streams 
of stars travelling in opposite directions. In both 
these great streams, the individual bodies are found, 
besides, to be alike in design, alike in chemical con- 
stitution, and alike in the stage of their development. 

A fable related by the Persian astronomer, Al Sufi 
(tenth century, A.D.) shows well the changes in the 
face of the sky which proper motions are bound to 
produce after great lapses of time. According to this 
fable the stars Sirius and Procyon were the sisters of 

284 



The Stars 

the star Canopus. Canopus married Rigel (another 
star,) but, having murdered her, he fled towards the 
South Pole, fearing the anger of his sisters. The 
fable goes on to relate, among other things, that 
Sirius followed him across the Milky Way. Mr. J. 
E. Gore, in commenting on the story, thinks that it 
may be based upon a tradition of Sirius having been 
seen by the men of the Stone Age on the opposite 
side of the Milky Way to that on which it now is. 

Sirius is in that portion of the heavens from which 
the sun is advancing. Its proper motion is such that 
it is gaining upon the earth at the rate of about ten 
miles per second, and so it must overtake the sun 
after the lapse of great ages. Vega, on the other 
hand, is coming towards us from that part of the 
sky towards which the sun is travelling. It should 
be about half a million years before the sun and Vega 
pass by one another. Those who have specially in- 
vestigated this question say that, as regards the 
probability of a near approach, it is much more 
likely that Vega will be then so far to one side of 
the sun, that her brightness will not be much greater 
than it is at this moment. 

Considerations like these call up the chances of 
stellar collisions. Such possibilities need not, how- 
ever, give rise to alarm ; for the stars, as a rule, are 
at such great distances from each other, that the pro- 
bability of relatively near approaches is slight. 

We thus see that the constellations do not in effect 
exist, and that there is in truth no real background 
to the sky. We find further that the stars are strewn 
through space at immense distances from each other, 
and are moving in various directions hither and 

28s 



Astronomy of To-day 

thither. The sun, which is merely one of them, is 
moving also in a certain direction, carrying the solar 
system along with it. It seems, therefore, but natural 
to suppose that many a star may be surrounded by 
some planetary system in a way similar to ours, 
which accompanies it through space in the course 
of its celestial journeyings. 



286 



CHAPTER XXIII 

THE STARS— continued 

The stars appear to us to be scattered about the sky 
without any orderly arrangement. Further, they are 
of varying degrees of brightness ; some being ex- 
tremely brilliant, whilst others can but barely be 
seen. The brightness of a star may arise from either 
of two causes. On the one hand, the body may be 
really very bright in itself; on the other hand, it 
may be situated comparatively near to us. Some- 
times, indeed, both these circumstances may come 
into play together. 

Since variation in brightness is the most noticeable 
characteristic of the stars, men have agreed to class 
them in divisions called ^^ magnitudes." This term, 
it must be distinctly understood, is employed in such 
classification without any reference whatever to actual 
size, being merely taken to designate roughly the 
amount of light which we receive from a star. The 
twenty brightest stars in the sky are usually classed 
in the first magnitude. In descending the scale, each 
magnitude will be noticed to contain, broadly speak- 
ing, three times as many stars as the one immediately 
above it. Thus the second magnitude contains 65, 
the third 190, the fourth 425, the fifth iioo, and the 
sixth 3200. The last of these magnitudes is about the 
limit of the stars which we are able to see with the 

287 



Astronomy of To-day 

naked eye. Adding, therefore, the above numbers 
together, we find that, without the aid of the tele- 
scope, we cannot see more than about 5000 stars in 
the entire sky — northern and southern hemispheres 
included. Quite a small telescope will, however, 
allow us to see down to the ninth magnitude, so 
that the total number of stars visible to us with 
such very moderate instrumental means will be well 
GJ/er 100,000. 

It must not, however, be supposed that the stars 
included within each magnitude are all of exactly the 
same brightness. In fact, it would be difficult to. say 
if there exist in the whole sky two stars which send 
us precisely the same amount of light. In arranging 
the magnitudes, all that was done was to make certain 
broad divisions, and to class within them such stars 
as were much on a par with regard to brightness. It 
may here be noted that a standard star of the first 
magnitude gives us about one hundred times as much 
light as a star of the sixth magnitude, and about one 
million times as much as one of the sixteenth magni- 
tude — which is near the limit of what we can see with 
the very best telescope. 

Though the first twenty stars in the sky are popularly 
considered as being of the first magnitude, yet several 
of them are much brighter than an average first 
magnitude star would be. For instance, Sirius — 
the brightest star in the whole sky — is equal to about 
eleven first magnitude stars, like, say, Aldebaran. In 
consequence of such differences, astronomers are 
agreed in^.classifying the brightest of them as brighter 
than the standard first magnitude star. On this prin- 
ciple Sirius would be about two and a half magnitudes 

288 



The Stars 

above the first. This notation is usefully employed in 
making comparisons between the amount of light 
which we receive from the sun, and that which we 
get from an individual star. Thus the sun will be 
about twenty-seven and a half magnitudes above the 
first magnitude. The range, therefore, between the 
light which we receive from the sun (considered 
merely as a very bright star) and the first magnitude 
stars is very much greater than that between the 
latter and the faintest star which can be seen with the 
telescope, or even registered upon the photographic 
plate. 

To classify stars merely by their magnitudes, without 
some (Jefinite note of their relative position in the sky, 
would be indeed of little avail. We must have some 
simple method of locating them in the memory, and 
the constellations of the ancients here happily come 
to our aid. A system combining magnitudes with 
constellations was introduced by Bayer in 1603, and 
is still adhered to. According to this the stars in each 
constellation, beginning with the brightest star, ?.re 
designated by the letters of the Greek alphabet taken 
in their usual order. For example, in the constellation 
of Canis Major, or the Greater Dog, the brightest star 
is the well-known Sirius, called by the ancients the 
^' Dog Star " ; and this star, in accordance with Bayer's 
method, has received the Greek letter a (alpha)^ and 
is consequently known as Alpha Canis Majoris.^ As 
soon as the Greek letters are used up in this way the 
Roman alphabet is brought into requisition, after 
which recourse is had to ordinary numbers. 

1 Attention must here be drawn to the fact that the name of the con 
stellation is always put in the genitive case. 

289 T 



Astronomy of To-day 

Notwithstanding this convenient arrangement, some 
of the brightest stars are nearly always referred to 
by certain proper names given to them in old times. 
For instance, it is more usual to speak of Sirius, Arc- 
turus, Vega, Capella, Procyon, Aldebaran, Regulus, 
and so on, than of a Canis Majoris, a Bootis, a Lyrae, 
a Aurigae, a Canis Minoris, a Tauri, a Leonis, &c. &c. 

*in order that future generations might be able to 
ascertain what changes were taking place in the face 
of the sky, astronomers have from time to time drawn 
up catalogues of stars. These lists have included stars 
of a certain degree of brightness, their positions in 
the sky being noted with the utmost accuracy possible 
at the period. The earliest known catalogue of this 
kind was made, as we have seen, by the celebrated 
Greek astronomer, Hipparchus, about the year 125 B.C. 
It contained 1080 stars. It was revised and brought 
up to date by Ptolemy in a.d. 150. Another celebrated 
list was that drawn up by the Persian astronomer, 
Al Sufi, about the year a.d. 964. In it 1022 stars were 
noted down. A catalogue of 1005 stars was made in 
1580 by the famous Danish astronomer, Tycho Brahe. 
Among modern catalogues that of Argelander (1799- 
1875) contained as many as 324,198 stars. It was ex- 
tended by Schonfeld so as to include a portion of the 
Southern Hemisphere, in which way 133,659 more 
stars were added. 

In recent years a project was placed on foot of 
making a photographic survey of the sky, the work 
to be portioned out among various nations. A great 
part of this work has already been brought to a con- 
clusion. About 15,000,000 stars will appear upon the 
plates ; but, so far, it has been proposed to catalogue 

290 



The Stars 

only about a million and a quarter of the brightest of 
them. This idea of surveying the face of the sky by 
photography sprang indirectly from the fine photo- 
graphs which Sir David Gill took, when at the Cape 
of Good Hope, of the Comet of 1882. The immense 
number of star-images which had appeared upon his 
plates suggested the idea that photography could be 
very usefully employed to register the relative positions 
of the stars. 

The arrangement of seven stars known as the 
" Plough " is perhaps the most familiar configuration 
in the sky (see Plate XIX., p. 292). In the United 
States it is called the '^ Dipper/' on account of its like- 
ness to the outline of a saucepan, or ladle. ^' Charles' 
Wain " was the old English name for it, and readers 
of Caesar will recollect it under SeptentrioneSy or the 
^' Seven Stars," a term which that writer uses as a 
synonym for the North. Though identified in most 
persons' minds with Ursa Major, or the Great Bear, 
the Plough is actually only a small portion of that 
famous constellation. Six out of the seven stars which 
go to make up the well-known figure are of the second 
magnitude, while the remaining one, which is the 
middle star of the group, is of the third. 

The Greek letters, as borne by the individual stars 
of the Plough, are a plain transgression of Bayer's 
method as above described, for they have certainly 
not been allotted here in accordance with the proper 
order of brightness. For instance, the third magni- 
tude star, just alluded to as being in the middle of 
the group, has been marked with the Greek letter h 
(Delta) ; and so is made to take rank before the stars 
composing what is called the ^< handle " of the Plough, 

291 



Astronomy of To-day 

which are all of the second magnitude. Sir William 
Herschel long ago drew attention to the irregular 
manner ia which Bayer's system had been applied. 
It is, indeed, a great pity that this notation was not 
originally worked out with greater care and correct- 
ness ; for, were it only reliable, it would afford great 
assistance to astronomers in judging of what changes 
in relative brightness have taken place among the 
stars. 

Though we may speak of using the constellations 
as a method of. finding our way about the sky, it is, 
however, to certain marked groupings in them of the 
brighter stars that we look for our sign-posts. 

Most of the constellations contain a group or so 
of noticeable stars, whose accidental arrangement 
dimly recalls the outline of some familiar geometrical 
figure and thus arrests the attention.^ For instance, 
in an almost exact line with the two front stars of 
the Plough, or ^' pointers " as they are called,^ and at 
a distance about five times as far away as the interval 
between them, there.will be found a third star of the 
second magnitude. This is known as Polaris, or the 
Pole Star, for it very nearly occupies that point of 
the heaven towards which the north pole of the earth's 

^ The early peoples, as we have seen, appear to have been attracted by 
those groupings of the stars which reminded them in a way of the figures 
of men and animals. We moderns, on the other hand, seek almost 
instinctively for geometrical arrangements. This iS; perhaps, symptomatic 
of the evolution of the race. In the growth of the individual we find, 
for example, something analogous. A child, who has been given pencil 
and paper, is almost certain to produce grotesque drawings of men and 
animals ; whereas the idle and half-conscious scribblings which a man 
may make upon his blotting-paper are usually of a geometrical character, 

2 Because the line joining them points in the direction of the Pole 
Star. 

292 







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K 


















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w 


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c 


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U E ON 
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The Stars 

axis is at present directed (see Plate XIX., p. 292). 
Thus during the apparently daily rotation of the 
heavens, this star looks always practically stationary. 
It will, no doubt, be remembered how Shakespeare 
has put into the mouth of Julius Caesar these memor- 
able words : — 

" But I am constant as the northern star, 
Of whose true-fix'd and resting quality 
There is no fellow in the firmament." 

On account of the curvature of the earth's surface, 
the height at which the Pole Star is seen above the 
horizon at any place depends regularly upon the 
latitude ; that is to say, the distance of the place 
in question from the equator. For instance, at the 
north pole of the earth, where the latitude is greatest, 
namely, 90°, the Pole Star will appear directly over- 
head ; whereas in England, where the latitude is 
about 50°, it will be seen a little more than half way 
up the northern sky. At the equator, where the 
latitude is nil^ the Pole Star will be on the horizon 
due north. 

In consequence of its unique position, the Pole 
Star is of very great service in the study of the con- 
stellations. It is a kind of centre around which to 
hang our celestial ideas — a starting point, so to speak, 
in our voyages about the sky. 

According to the constellation figures, the Pole 
Star is in Ursa Minor, or the Little Bear, and is 
situated at the end of the tail of that imaginary figure 
(see Plate XIX., p. 292). The chief stars of this con- 
stellation form a group not unlike the Plough, except 
that the ^'handle" is turned in the contrary direction. 

293 



Astronomy of To-day 

The Americans, in consequence, speak of it as the 
'' Little Dipper." 

Before leaving this region of the sky, it will be 
well to draw attention to the second magnitude star J" 
in the Great Bear (Zeta Ursae Majoris), which is the 
middle star in the ^^ handle" of the Plough. This 
star is usually known as Mizar, a name given to it 
by the Arabians. A person with good eyesight can 
see quite near to it a fifth magnitude star, known 
under the name of Alcor. We have here a very good 
example of that deception in the estimation of objects 
in the sky, which has been alluded to in an earlier 
chapter. Alcor is indeed distant from Mizar by about 
one-third the apparent diameter of the moon, yet no 
one would think so I 

On the other side of Polaris from the Plough, 
and at about an equal apparent distance, will be 
found a figure in the form of an irregular '^ W ", 
made up of second and third magnitude stars. This 
is the well-known '^ Cassiopeia's Chair" — portion of 
the constellation of Cassiopeia (see Plate XIX., p. 292). 

On either side of the Pole Star, about midway 
between the Plough and Cassiopeia's Chair, but a 
little further off from it than these, are the constella- 
tions of Auriga and Lyra (see Plate XIX., p. 292). The 
former constellation will be easily recognised, be- 
cause its chief features are a brilliant yellowish first 
magnitude star, with one of the second magnitude 
not far from it. The first magnitude star is Capella, 
the other is /3 Aurigae. Lyra contains only one first 
magnitude star — Vega, pale blue in colour. This star 
has a certain interest for us from the fact that, as a 
consequence of that slow shift of direction of the 

294 



The Stars 

earth's axis known as Precession, it will be very near 
the north pole of the heavens in some 12,000 years, 
and so will then be considered the pole star (see 
Plate XIX., p. 292). The constellation of Lyra itself, 
it must also be borne in mind, occupies that region 
of the heavens towards which the solar system is 
travelling. 

The handle of the Plough points roughly towards 
the constellation of Bootes^ in which is the brilliant 
first magnitude star Arcturus. This star is of an 
orange tint. 

Between Bootes and Lyra lie the constellations of 
Corona Borealis (or the Northern Crown) and Hercules, 
The chief feature of Corona Borealis, which is a 
small constellation, is a semicircle of six small stars, 
the brightest of which is of the second magnitude. 
The constellation of Hercules is very extensive, but 
contains no star brighter than the third magnitude. 

Near to Lyra, on the side away from Hercules, are 
the constellations of Cygnus and Aquila. Of the two, 
the former is the nearer to the Pole Star, and will be 
recognised by an arrangement of stars widely set in 
the form of a cross, or perhaps indeed more like the 
framework of a boy's kite. The position of Aquila 
will be found through the fact that three of its 
brightest stars are almost in a line and close together. 
The middle of these is Altair, a yellowish star of the 
first magnitude. 

At a little distance from Ursa Major, on the side 
away from the Pole Star, is the constellation of Leo, 
or the Lion. Its chief feature is a series of seven stars, 
supposed to form the head of that animal. The ar- 
rangement of these stars is, however, much more like 

295 



Astronomy of To-day 

a sickle^ wherefore this portion of the constellation is 
usually known as the '' Sickle of Leo." At the end 
of the handle of the sickle is a white first magnitude 
star — Regulus. 

The reader will, no doubt, recollect that it is from 
a point in the Sickle of Leo that the Leonid meteors 
appear to radiate. 

The star second in brightness in the constellation 
of Leo is known as Denebola. This star, now below 
the second magnitude, seems to have been very much 
brighter in the past. It is noted, indeed, as a brilliant 
first magnitude star by Al Sufi, that famous Persian 
astronomer who lived, as we have seen, in the tenth 
century. Ptolemy also notes it as of the first magni- 
tude. 

In the neighbourhood of Auriga, and further than 
it from the Pole Star, are several remarkable constel- 
lations — Taurus, Orion, Gemini, Canis Minor, and 
Canis Major (see Plate XX., p. 296). 

The first of these, Taurus (or the Bull), contains 
two conspicuous star groups — the Pleiades and the 
Hyades. The Pleiades are six or seven small stars 
quite close together, the majority of which are of the 
fourth magnitude. This group is sometimes occulted 
by the moon. The way in which the stars composing 
it are arranged is somewhat similar to that in the 
Plough, though of course on a scale ever so much 
smaller. The impression which the group itself gives 
to the casual glance is thus admirably pictured in 
Tennyson's Locksley Hall : — 

" Many a night I saw the Pleiads, rising through the mellow 
shade, 
Glitter like a swarm of fire-flies tangled in a silver braid." 
296 



The Stars 

The group of the Hyades occupies the '' head " of 
the Bull, and is much more spread out than that 
of the Pleiades. It is composed besides of brighter 
stars, the brightest being one of the first magnitude, 
Aldebaran. This star is of a red colour, and is some- 
times known as the ^^ Eye of the Bull." 

The constellation of Orion is easily recognised as an 
irregular quadrilateral formed of four bright stars, 
two of which, Betelgeux (reddish) and Rigel (brilliant 
white), are of the first magnitude. In the middle of 
the quadrilateral is a row of three second magnitude 
stars, known as the ''Belt" of Orion. Jutting off 
from this is another row of stars called the '' Sword " 
of Orion. 

The constellation of Geminz, or the Twins, contains 
two bright stars — Castor and Pollux — close to each 
other. Pollux, though marked with the Greek letter 
P, is the brighter of the two, and nearly of the standard 
first magnitude. 

Just further from the Pole than Gemini, is the con- 
stellation of Cam's Minor, or the Lesser Dog. Its 
chief star is a white first magnitude one — Procyon. 

Still further again from the Pole than Canis Minor 
is the constellation of Canis Major, or the Greater 
Dog. It contains the brightest star in the whole sky, 
the first magnitude star Sirius, bluish-white in colour, 
also known as the '' Dog Star." This slar is almost 
in line with the stars forming the Belt of Orion, and 
is not far from that constellation. 

Taken in the following order, the stars Capella, p 
Aurigae, Castor, Pollux, Procyon, and Sirius, when 
they are all above the horizon at the same time, form 
a beautiful curve stretching across the heaven. 

297 



Astronomy of To-day 

The groups of stars visible in the southern skies 
have by no means the same fascination for us as 
those in the northern. The ancients were in general 
unacquainted with the regions beyond the equator, 
and so their scheme of constellations did not include 
the sky around the South Pole of the heavens. In 
modern times, however, this part of the celestial 
expanse was also portioned out into constellations 
for the purpose of easy reference ; but these group- 
ings plainly lack that simplicity of conception and 
legendary interest which are so characteristic of the 
older ones. 

The brightest star in the southern skies is found in 
the constellation of Argo, and is known as Canopus. 
In brightness it comes next to Sirius, and so is second 
in that respect in the entire heaven. It does not, 
however, rise above the English horizon. 

Of the other southern constellations, two call for 
especial notice, and these adjoin each other. One is 
Centaurus (or the Centaur), which contains the two 
first magnitude stars, a and ^ Centauri. The first of 
these. Alpha Centauri, comes next in brightness to 
Canopus, and is notable as being the nearest of all the 
stars to our earth. The other constellation is called 
Crux, and contains five stars set in the form of a 
rough cross, known as the ^^ Southern Cross." The 
brightest of these, a Crucis, is of the first magnitude. 

Owing to the Precession of the Equinoxes, which, 
as we have seen, gradually shifts the position of the 
Pole among the stars, certain constellations used to 
be visible in ancient times in more northerly latitudes 
than at present. For instance, some five thousand 
years ago the Southern Cross rose above the English 

298 



The Stars 

horizon^ and was just visible in the latitude of London. 
It has, however, long ago even ceased to be seen in 
the South of Europe. The constellation of Crux 
happens to be situated in that remarkable region of 
thie southern skies, in which are found the stars 
Canopus and Alpha Centauri, and also the most 
brilliant portion of the Milky Way. It is believed to 
be to this grand celestial region that allusion is made 
in the Book of Job (ix. 9), under the title of the 
^•Chambers of the South." The ''Cross" must have 
been still a notable feature in the sky of Palestine in 
the days when that ancient poem was written. 

There is no star near enough to the southern pole 
of the heavens to earn the distinction of South Polar 
Star. 

The Galaxy, or Milky Way (see Plate XX., p. 296), is 
a broad band of diffused light which is seen to stretch 
right around the sky. The telescope, however, shows 
it to be actually composed of a great host of very faint 
stars — too faint, indeed, to be separately distinguished 
with the naked eye. Along a goodly stretch of its 
length it is cleft in two ; while near the south pole of 
the heavens it is entirely cut across by a dark streak. 

In this rapid survey of the face of the sky, we have 
not been able to do more than touch in the broadest 
manner upon some of the most noticeable star groups 
and a few of the most remarkable stars. To go any 
further is not a part of our purpose ; our object being 
to deal with celestial bodies as they actually are, and 
not in those groupings under which they display them- 
selves to us as a mere result of perspective. 



299 



CHAPTER XXIV 

SYSTEMS OF STARS 

Many stars are seen comparatively close together. 
This may plainly arise from two reasons. Firstly, the 
stars may happen to be almost in the same line of 
sight ; that is to^say, seen in nearly the same direction ; 
and though one star may be ever so much nearer to 
us than the other, the result will give all the appear- 
ance of a related pair. A seeming arrangement of 
two stars in this way is known as a '' double," or 
double star ; or, indeed, to be very precise, an ^^ optical 
double." Secondly, in a pair of stars, both bodies 
may be about the same distance from us, and actually 
connected as a system like, for instance, the moon 
and the earth. A pairing of stars in this way, though 
often casually alluded to as a double star, is properly 
termed a "binary," or binary system. 

But collocations of stars are by no means limited 
to two. We find, indeed, all over the sky such ar- 
rangements in which there are three or more stars ; 
and these are technically known as ^ triple " or 
"multiple" stars respectively. Further, groups are 
found in which a great number of stars are closely 
massed together, such a massing together of stars 
being known as a ^'cluster." 

The Pole Star (Polaris) is a double star, one of the 
components being of a little below the second magni- 

300 



Systems of Stars 



tudc; and the other a little below the ninths They 
are so close together that they appear as one star to 
the naked eye, but they may be seen separate with 
a moderately sized telescope. The brighter star is 
yellowish, and the faint one white. This brighter star 
is found by means of the spectroscope to be actually 
composed of three stars so very close together that 
they cannot be seen separately even with a telescope. 
It is thus a triple star, and the three bodies of which 
it is composed are in circulation about each other. 
Two of them are darker than the third. 

The method of detecting binary stars by means of 
the spectroscope is an application of Doppler's prin- 
ciple. It will, no doubt, be remembered that, according 
to the principle in question, we are enabled, from cer- 
tain shiftings of the lines in the spectrum of a luminous 
body, to ascertain whether that body is approaching 
us or receding from us. Now there are certain stars 
which always appear single even in the largest 
telescopes, but when the spectroscope is directed to 
them a spectrum with two sets of lines is seen. Such 
stars must, therefore, be double. Further, if the 
shiftings of the lines, in a spectrum like this, tell us 
that the component stars are making small movements 
to and from us which go on continuously, we are 
therefore justified in concluding that these are the 
orbital revolutions of a binary system greatly com- 
pressed by distance. Such connected pairs of stars, 
since they cannot be seen separately by means of any 
telescope, no matter how large, are known as ^' spectro- 
scopic binaries." 

In observations of spectroscopic binaries we do not 
always get a double spectrum. Indeed, if one of the 

301 



Astronomy of To-day 

ccHnponents be below a certain magnitude, its spectrum 
will not appear at all ; and so we are left in the 
strange uncertainty as to whether this component is 
merely faint or actually dark. It is, however, from 
the shiftings of the lines in the spectrum of the other 
component that we see that an orbital movement is 
going on, and are thus enabled to conclude that two 
bodies are here connected into a system, although 
one of these bodies resolutely refuses directly to re- 
veal itself even to the all-conquering spectroscope. 

Mizar, that star in the handle of the Plough to 
which we have already drawn attention, will be found 
with a small telescope to be a fine double, one of 
the components being white and the other greenish. 
Actually, however, as the American astronomer, 
Professor F. R. Moulton, points out, these stars are 
so far from each other that if we could be transferred 
to one of them we should see the other merely as an 
ordinary bright star. The spectroscope shows that 
the brighter of these stars is again a binary system 
of two huge suns, the components revolving around 
each other in a period of about twenty days. This 
discovery made by Professor E. C. Pickering, the 
first of the kind by means of the spectroscope, was 
announced in 1889 from the Harvard Observatory 
in the United States. 

A star close to Vega, known as e (Epsilon) Lyrae 
(see Plate XIX., p. 292), is a double, the components 
of which may be seen separately with the naked eye 
by persons with very keen eyesight. If this star, 
however, be viewed with the telescope, the two com- 
panions will be seen far apart ; and it will be noticed 
that each of them is again a double. 

302 



Systems of Stars 



By means of the spectroscope Capella is shown to 
be really composed of two stars (one about twice as 
bright as the other) situated very close together and 
forming a binary system. Sirius is also a binary 
system ; but it is what is called a '* visual " one, for 
its component stars may be seen separately in very 
large telescopes. Its double, or rather binary, nature, 
was discovered in 1862 by the celebrated optician 
Alvan G. Clark, while in the act of testing the 18-inch 
refracting telescope, then just constructed by his firm, 
and now at the Dearborn Observatory, Illinois, U.S.A. 
The companion is only of the tenth magnitude, and 
revolves around Sirius in a period of about fifty years, 
at a mean distance equal to about that of Uranus 
from the sun. Seen from Sirius, it would shine 
only something like our full moon. It must be 
self-luminous and not a mere planet ; for Mr. Gore 
has shown that if it shone only by the light re- 
flected from Sirius, it would be quite invisible even 
in the Great Yerkes Telescope. 

Procyon is also a binary, its companion having 
been discovered by Professor J. M. Schaeberle at the 
Lick Observatory in 1896. The period of revolution 
in this system is about forty years. Observations by 
Mr. T. Lewis of Greenwich seem, however, to point to 
the companion being a small nebula rather than a star. 

The star 97 (Eta) Cassiopeiae (see Plate XIX., p. 292), 
is easily seen as a fine double in telescopes of 
moderate size. It is a binary system, the com- 
ponent bodies revolving around their common centre 
of gravity in a period of about two hundred years. 
This system is comparatively near to us, i.e, about 
nine light years, or a little further off than Sirius. 

303 



Astronomy of To-day 

In a small telescope the star Castor will be found 
double, the components, one of which is brighter 
than the other, forming a binary system. The fainter 
of these was found by Belopolsky, with the spectro- 
scope, to be composed of a system of two stai one 
bright and the other either dark or not so bright, 
revolving around each other in a period of about 
three days. The brighter component of Castor is 
also a spectroscopic bin :y, with a period of about 
nine days ; so that the vvhole of what we see with 
the naked eye as Castor, is in reality a remarkable 
system of four stars in mutual orbital movement. 

Alpha Centauri — the nearest star to the earth — is 
a visual binary, the component bodies revolving 
around each other in a period of about eighty-one 
years. The extent of this system is about the same 
as that of Sirius. Viewed from each other, the bodies 
would shine only like our sun as seen from Neptune. 

Among the numerous binary stars the orbits of 
some fifty have been satisfactorily determined. Many 
double stars, for which this has not yet been done, 
are, however, believed to be, without doubt, binary. 
In some cases a parallax has been found ; so that 
we are enabled to estimate in miles the actual extent 
of such systems, and the masses of the bodies in 
terms of the sun's mass. 

Most of the spectroscopic binaries appear to be 
upon a smaller scale than the telescopic ones. Some 
are, indeed, comparatively speaking, quite small. 
For instance, the component stars forming /3 Aurigae 
are about eight million miles apart, while in f Gemi- 
norum, the distance between the bodies is only a 
little more than a million miles. 

304 



Systems of Stars 



spectroscopic binaries are probably very numerous. 
Professor W. W. Campbell, Director of the Lick 
Observatory, estimates, for instance, that, out of about 
every half-a-dozen stars, one is a spectroscopic 
binary. 

It is only in the case of binary systems that we 
can discover the masses of stars at all. These are 
ascertained from their movements with regard to 
each other under the influence of their mutual 
gravitative attractions. In the case of simple stars 
we have clearly nothing of the kind to judge by ; 
though, if we can obtain a parallax, we may hazard 
a guess from their brightness. 

Binary stars were incidentally discovered by Sir 
William Herschel. In his researches to get a stellar 
parallax he had selected a number of double stars 
for test purposes, on the assumption that, if one of 
such a pair were much nearer than the other, it might 
show a displacement with regard to its neighbour 
as a direct consequence of the earth's orbital move- 
ment around the sun. He, however, failed entirely 
to obtain any parallaxes, the triumph in this being, 
as we have seen, reserved for Bessel. But in some 
of the double stars which he had selected, he found 
certain alterations in the relative positions of the 
bodies, which plainly were not a consequence of the 
earth's motion, but showed rather that there was 
an actual circling movement of the bodies them- 
selves under their mutual attractions. It is to be 
noted that the existence of such connected pairs had 
been foretold as probable by the Rev. John Michell, 
who Hved a short time before Herschel. 

The researches into binary systems — both those 

305 u 



Astronomy of To-day 

which can be seen with the eye and those which 
can be observed by means of the spectroscope, 
ought to impress upon us very forcibly the wide 
sway of the law of gravitation. 

Of star clusters about loo are known, and such 
systems often contain several thousand stars. They 
usually cover an area of sky somewhat smaller than 
the moon appears to fill. In most clusters the stars 
are very faint, and, as a rule, are between the twelfth 
and sixteenth magnitudes. It is difficult to say 
whether these are actually small bodies, or whether 
their faintness is due merely to their great distance 
from us, since they are much too far off to show any 
appreciable parallactic displacement. Mr. Gore, how- 
ever, thinks there is good evidence to show that the 
stars in clusters are really close, and that the clusters 
themselves fill a comparatively small space. 

One of the finest examples of a cluster is the great 
globular one, in the constellation of Hercules, dis- 
covered by Halley in 1714. It contains over 5000 
stars, and upon a clear, dark night is visible to the 
naked eye as a patch of light. In the telescope, 
however, it is a wonderful object. There are also 
fine clusters in the constellations of Auriga, Pegasus, 
and Canes Venatici. In the southern heavens there 
are some magnificent examples of globular clusters. 
This hemisphere seems, indeed, to be richer in such 
objects than the northern. For instance, there is 
a great one in the constellation of the Centaur, con- 
taining some 6000 stars (see Plate XXI., p. 306). 

Certain remarkable groups of stars, of a nature 
similar to clusters, though not containing such faint 
or densely packed stars as those we have just alluded 

306 




Plate XXI. The Great Globular Cluster in the Southern 
Constellation of Centaurus 



From a photograph taken at the Cape Observatory, on May 241 
Time of exposure, i hour. 



1905. 

(Page 306) 



Systems of Stars 

to, call for a mention in this connection. The best 
example of such star groups are the Pleiades and 
the Hyades (see Plate XX., p. 296), Coma Berenices, 
and Praesepe (or the Beehive), the last-named being 
in the constellation of Cancer. 

Stars which alter in their brightness are called 
Variable Stars, or ^^ variables." The first star whose 
variability attracted attention is that known as 
Omicron Ceti, namely, the star marked with the 
Greek letter (Omicron) in the constellation of Cetus, 
or the Whale, a constellation situated not far from 
Taurus. This star, the variability of which was 
discovered by Fabricius in 1596, is also known as 
Mira, or the '* Wonderful," on account of the extra- 
ordinary manner in which its light varies from time 
to time. The star known by the name of Algol,i 
popularly called the ^' Demon Star "-—whose astro- 
nomical designation is p (Beta) Persei, or the star 
second in brightness in the constellation of Perseus — 
was discovered by Goodricke, in the year 1783, to be 
a variable star. In the following year /3 Lyrae, the 
star in Lyra next in order of brightness after Vega, 
was also found by the same observer to be a variable. 
It may be of interest to the reader to know that 
Goodricke was deaf and dumb, and that he died 
in 1786 at the early age of twenty-one years ! 

It was not, however, until the close of the nineteenth 
century that much attention was paid to variable 
stars. Now several hundreds of these are known, 
thanks chiefly to the observations of, amongst others, 

^ The name Al gul, meaning the Demon, was what the old Arabian 
astronomers called it, which looks very much as if they had already 
noticed its rapid fluctuations in brightness. 



Astronomy of To-day 

Professor S. C. Chandler of Boston, U.S.A., Mr. John 
EUard Gore of Dublin, and Dr. A. W. Roberts of 
South Africa. This branch of astronomy has not, 
indeed, attracted as much popular attention as it 
deserves, no doubt because the nature of the work 
required does not call for the glamour of an observa- 
tory or a large telescope. 

The chief discoveries with regard to variable stars 
have been made by the naked eye, or with a small 
binocular. The amount of variation is estimated by 
a comparison with other stars. As in many other 
branches of astronomy, photography is now em- 
ployed in this quest with marked success ; and 
lately many variable stars have been found to exist 
in clusters and nebulae. 

It was at one time considered that a variable star 
was in all probability a body, a portion of whose 
surface had been relatively darkened in some manner 
akin to that in which sun spots mar the face of the 
sun ; and that when its axial rotation brought the less 
illuminated portions in turn towards us, we witnessed a 
consequent diminution in the star's general brightness. 
Herschel, indeed, inclined to this explanation, for his 
belief was that all the stars bore spots like those of 
the sun. It appears preferably thought nowadays 
that disturbances take place periodically in the at- 
mosphere or surroundings of certain stars, perhaps 
through the escape of imprisoned gases, and that 
this may be a fruitful cause of changes of brilliancy. 
The theory in question will, however, apparently ac- 
count for only one class of variable star, namely, 
that of which Mira Ceti is the best-known example. 
The scale on which it varies in brightness is very 

308 



Systems of Stars 

great, for it changes from the second to the ninth 
magnitude. For the other leading type of variable 
star, Algol, of which mention has already been made, 
is the best instance. The shortness of the period in 
which the changes of brightness in such stars go 
their round, is the chief characteristic of this latter 
class. The period of Algol is a little under three 
days. This star when at its brightest is of about 
the second magnitude, and when least bright is re- 
duced to below the third magnitude ; from which 
it follows that its light, when at the minimum, is 
only about one-third of what it is when at the 
maximum. It seems definitely proved by means of 
the spectroscope that variables of this kind are 
merely binary stars, too close to be separated by 
the telescope, which, as a consequence of their orbits 
chancing to be edgewise towards us, eclipse each 
other in turn time after time. If, for instance, both 
components of such a pair are bright, then when one 
of them is right behind the other, we will not, of 
course, get the same amount of light as when they 
are side by side. If, on the other hand, one of the 
components happens to be dark or less luminous and 
the other bright, the manner in which the light of 
the bright star will be diminished when the darker 
star crosses its face should easily be understood. It 
is to the second of these types that Algol is supposed 
to belong. The Algol system appears to be com- 
posed of a body about as broad as our sun, which 
regularly eclipses a brighter body which has a diameter 
about half as great again. 

Since the companion of Algol is often spoken of 
as a dark body, it were well here to point out that 

309 



Astronomy of To-day 

we have no evidence at all that it is entirely devoid 
of light. We have already found, in dealing with 
spectroscopic binaries, that when one of the com- 
ponent stars is below a certain magnitude^ its 
spectrum will not be seen ; so one is left in the 
glorious uncertainty as to whether the body in 
question is absolutely dark, or darkish, or faint, or 
indeed only just out of range of the spectroscope. 

It is thought probable by good authorities that the 
companion of Algol is not quite dark, but has some 
inherent light of its own. It is, of course, much too 
near Algol to be seen with the largest telescope. 
There is in fact a distance of only from two to three 
millions of miles between the bodies, from which 
Mr. Gore infers that they would probably remain un- 
separated even in the largest telescope which could 
ever be constructed by man. 

The number of known variables of the Algol type 
is, so far, small ; not much indeed over thirty. In 
some of them the components are believed to re- 
volve touching each other, or nearly so. An extreme 
example of this is found in the remarkable star V. 
Puppis, an Algol variable of the southern hemisphere. 
Both its components are bright, and the period of 
light variation is about one and a half days. Dr. A. 
W. Roberts finds that the bodies are revolving around 
each other in actual contact, 

Te^nporary stars 3.ve stars which have suddenly blazed 
out in regions of the sky where no star was previously 
seen, and have faded away more or less gradually. 

It was the appearance of such a star, in the year 

1 Mr. Gore thinks that the companion of Algol may be a star of the 
sixth magnitude. 

310 



Systems of Stars 



134 B.C., which prompted Hipparchus to make his 
celebrated catalogue, with the object of leaving a 
record by which future observers could note celestial 
changes. In 1572 another star of this kind flashed 
out in the constellation of Cassiopeia (see Plate XIX., 
p. 292), and was detected by Tycho Brahe. It became 
as bright as the planet Venus, and eventually was 
visible in the daytime. Two years later, however, 
it disappeared, and has never since been seen. In 
1604 Kepler recorded a similar star in the constella- 
tion of Ophiuchus which grew to be as bright as 
Jupiter. It also lasted for about two years, and then 
faded away, leaving no trace behind. It is rarely, how- 
ever, that temporary stars attain to such a brilliance ; 
and so possibly in former times a number of them 
may have appeared, but not have risen to a sufficient 
magnitude to attract attention. Even now, unless 
such a star becomes clearly visible to the naked eye, 
it runs a good chance of not being detected. A curious 
point, worth noting, with regard to temporary stars 
is that the majority of them have appeared in the 
Milky Way. 

These sudden visitations have in our day received 
the name of Novcb ; that is to say, ^' New " Stars. Two, 
in recent years, attracted a good deal of attention. 
The first of these, known as Nova Aurigae, or the 
New Star in the constellation of Auriga, was dis- 
covered by Dr. T. D. Anderson at Edinburgh in 
January 1892. At its greatest brightness it attained 
to about the fourth magnitude. By April it had sunk 
to the twelfth, but during August it recovered to the 
ninth magnitude. After this last flare-up it gradually 
faded away. 

311 



Astronomy of To-day 

The startling suddenness with which temporary 
stars usually spring into being is the groundwork 
upon which theories to account for their origin have 
been erected. That numbers of dark stars, extin- 
guished suns, so to speak, may exist in space, there 
is a strong suspicion ; and it is just possible that we 
have an instance of one dark stellar body in the 
companion of Algol. That such dark stars might 
be in rapid motion is reasonable to assume from 
the already known movements of bright stars. Two 
dark bodies might, indeed, coUide together, or a 
collision might take place between a dark star and a 
star too faint to be seen even with the most powerful 
telescope. The conflagration produced by the im- 
pact would thus appear where nothing had been seen 
previously. Again, a similar effect might be produced 
by a dark body, or a star too faint to be seen, being 
heated to incandescence by plunging in its course 
through a nebulous mass of matter, of which there 
are many examples lying about in space. 

The last explanation, which is strongly reminiscent 
of what takes place in shooting stars, appears more 
probable than the collision theory. The flare-up of 
new stars continues, indeed, only for a comparatively 
short time ; whereas a collision between two bodies 
would, on the other hand, produce an enormous nebula 
v/hich might take even millions of years to cool down. 
We have, indeed, no record of any such sudden ap- 
pearance of a lasting nebula. 

The other temporary star, known as Nova Persei, 
or the new star in the constellation of Perseus, 
was discovered early in the morning of February 22, 
1901, also by Dr. Anderson. A day later it had 

312 



Systems of Stars 



grown to be brighter than Capella. Photographs 
which had been taken, some three days previous to 
its discovery, of the very region of the sky in which 
it had burst forth, were carefully examined, and it 
was not found in these. At the end of two days after 
its discovery Nova Persei had lost one-third of 
its light. During the ensuing six months it passed 
through a series of remarkable fluctuations, varying 
in brightness between the third and fifth magnitudes. 
In the month of August it was seen to be surrounded 
by luminous matter in the form of a nebula, which 
appeared to be gradually spreading to some distance 
around. Taking into consideration the great way off 
at which all this was taking place, it looked as if 
the new star had ejected matter which was travelling 
outward with a velocity equivalent to that of light. 
The remarkable theory was, however, put forward 
by Professor Kapteyn and the late Dr. W. E. Wilson 
that there might be after all no actual transmission 
of matter ; but that perhaps the real explanation was 
the gradual illumination of hitherto invisible nebulous 
matter, as a consequence of the fiare-up which had 
taken place about six months before. It was, there- 
fore, imagined that some dark body moving through 
space at a very rapid rate had plunged through a mass 
of invisible nebulous matter, and had consequently 
become heated to incandescence in its passage, very 
much like what happens to a meteor when moving 
through our atmosphere. The illumination thus set 
up temporarily in one point, being transmitted through 
the nebulous wastes around with the ordinary velo- 
city of light, had gradually rendered this surrounding 
matter visible. On the assumptions required to fit in 

313 



Astronomy of To-day 

with such a theory, it was shown that Nova Persei 
must be at a distance from which Hght would take 
about three hundred years in coming to us. The actual 
outburst of illumination, which gave rise to this tem- 
porary star, would therefore have taken place about 
the beginning of the reign of James I. 

Some recent investigations with regard to Nova 
Persei have, however, greatly narrowed down the 
above estimate of its distance from us. For instance, 
Bergstrand proposes a distance of about ninety-nine 
light years ; while the conclusions of Mr. F. W. 
Very would bring it still nearer, i,e, about sixty-five 
light years. 

The last celestial objects with which we have here 
to deal are the Nebulce. These are masses of diffused 
shining matter scattered here and there through the 
depths of space. Nebulae are of several kinds, and 
have been classified under the various headings of 
Spiral, Planetary, Ring, and Irregular. 

A typical spiral nobuldi is composed of a disc-shaped 
central portion, with long curved arms projecting 
from opposite sides of it, which give an impression 
of rapid rotatory movement. 

The discovery of spiral nebulae was made by Lord 
Rosse with his great 6-foot reflector. Two good ex- 
amples of these objects will be found in Ursa Major, 
while there is another fine one in Canes Venatici (see 
Plate XXII., p. 314); a constellation which Hes between 
Ursa Major and Bootes. But the finest spiral of all, 
perhaps the most remarkable nebula known to us, is 
the Great Nebula in the constellation of Andromeda, 
(see Plate XXIII., p. 316) — a constellation just further 
from the pole than Cassiopeia. When the moon is 

314 




Plate XXII. Si'ikal Nebula in the Constellation of Canes 
Venatici 

From a photograph by the late Dr. W. E. Wilson. D.Sc, F.R.S. 

(Page 3I^) 



Systems of Stars 



absent and the night clear this nebula can be easily 
seen with the naked eye as a small patch of hazy 
Hght. It is referred to by Al Sufi. 

Spiral nebulae are white in colour, whereas the 
other kinds of nebula have a greenish tinge. They 
are also by far the most numerous ; and the late 
Professor Keeler, who considered this the normal 
type of nebula, estimated that there were at least 
120,000 of such spirals within the reach of the Crossley 
reflector of the Lick Observatory. Professor Perrine 
has indeed lately raised this estimate to half a million, 
and thinks that with more sensitive photographic 
plates and longer exposures the number of spirals 
would exceed a million. The majority of these objects 
are very small, and appear to be distributed over the 
sky in a fairly uniform manner. 

Planetary nebulae are small faint roundish objects 
which, when seen in the telescope, recall the appear- 
ance of a planet, hence their name. One of these 
nebulae, known astronomically as G.C. 4373, has 
recently been found to be rushing through space to- 
wards the earth at a rate of between thirty and forty 
miles per second. It seems strange, indeed, that any 
gaseous mass should move at such a speed ! 

What are known as ring nebulae were until recently 
believed to form a special class. These objects have 
the appearance of mere rings of nebulous matter. 
Much doubt has, however, been thrown upon their 
being rings at all ; and the best authorities regard 
them merely as spiral nebulae, of which we happen 
to get a foreshortened view. Very few examples are 
known, the most famous being one in the constella- 
tion of Lyra, usually known as the Annular Nebula 

315 



Astronomy of To-day 

in Lyra. This object is so remote from us as to 
be entirely invisible to the naked eye. It contains 
a star of the fifteenth magnitude near to its centre. 
From photographs taken with the Crossley reflector, 
Professor Schaeberle finds in this nebula evidences of 
spiral structure. It may here be mentioned that the 
Great Nebula in Andromeda, which has now turned 
out to be a spiral, had in earlier photographs the 
appearance of a ring. 

There also exist nebulae of irregular form, the most 
notable being the Great Nebula in the constellation of 
Orion (see Plate XXIV., p. 318). It is situated in the 
centre of the '' Sword " of Orion (see Plate XX,, p. 296). 
In large telescopes it appears as a magnificent object, 
and in actual dimensions it must be much on the same 
scale as the Andromeda Nebula. The spectroscope 
tells us that it is a mass of glowing gas. 

The Trifid Nebula, situated in the constellation of 
Sagittarius, is an object of very strange shape. Three 
dark clefts radiate from its centre, giving it an appear- 
ance as if it had been torn into shreds. 

The Dumb-bell Nebula, a celebrated object, so called 
from its hkeness to a dumb-bell, turns out, from recent 
photographs taken by Professor Schaeberle, which 
bring additional detail into view, to be after all a 
great spiral. 

There is a nest, or rather a cluster of nebula in the 
constellation of Coma Berenices ; over a hundred of 
these objects being here gathered into a space of sky 
about the size of our full moon. 

The spectroscope informs us that spiral nebulae are 
composed of partially-cooled matter. Their colour, 
as we have seen, is white. Nebulae of a greenish tint 

316 




Plate XXIII. The Gueat Nebula in the Constellation of 
Andromeda 

From a photograpli taken at the Verkes Observatory. 

(Page 314) 



Systems of Stars 



are, on the other hand, found to be entirely in a 
gaseous condition. Just as the solar corona contains 
an unknown element, which for the time being has 
been called ''Coronium/' so do the gaseous nebulae 
give evidence of the presence of another unknown 
element. To this Sir William Huggins has given the 
provisional name of ^^ Nebulium." 

The Magellanic Clouds are two patches of nebulous- 
looking light, more or less circular in form, which are 
situated in the southern hemisphere of the sky. They 
bear a certain resemblance to portions of the Milky 
Way, but are, however, not connected with it. They 
have received their name from the celebrated navi- 
gator, Magellan, who seems to have been one of the 
first persons to draw attention to them. *^ Nubeculae " 
is another name by which they are known, the larger 
cloud being styled nubecula major and the smaller one 
nubecula minor. They contain within them stars, 
clusters, and gaseous nebulae. No parallax has yet 
been found for any object which forms part of the 
nubeculae, so it is very difficult to estimate at what 
distance from us they may lie. They are, however, 
considered to be well within our stellar universe. 

Having thus brought to a conclusion our all too 
brief review of the stars and the nebulae — of the lead- 
ing objects in fine which the celestial spaces have 
revealed to man — we will close this chapter with the 
recent summation by Sir David Gill of the relations 
which appear to obtain between these various bodies. 
'' Huggins's spectroscope," he says, *'has shown that 
many nebulae are not stars at all ; that many well- 
condensed nebulae, as well as vast patches of nebulous 
light in the sky, are but inchoate masses of luminous 

317 



Astronomy of To-day 

gas. Evidence upon evidence has accumulated to 
show that such nebulae consist of the matter out of 
which stars {i.e, suns) have been and are being evolved. 
The different types of star spectra form such a com- 
plete and gradual sequence (from simple spectra re- 
sembHng those of nebulae onwards through types of 
gradually increasing complexity) as to suggest that 
we have before us, written in the cryptograms of these 
spectra, the complete story of the evolution of suns 
from the inchoate nebula onwards to the most active 
sun (like our own), and then downward to the almost 
heatless and invisible ball. The period during which 
human life has existed upon our globe is probably 
too short — even if our first parents had begun the 
work — to afford observational proof of such a cycle 
of change in any particular star ; but the fact of such 
evolution, with the evidence before us, can hardly be 
doubted.'^ i 

^ Presidential Address to the British Association for the Advancement 
of Science (Leicester, 1907), by Sir David Gill, K.C.B., LL.D., F.R.S., 
&c. &c. 



318 




Plate XXIV. The Great Nebula in the Constellaiiox 

From a photograph taken at the ^'erkes Observatory. 



OF Orion 
(Page 316) 



CHAPTER XXV 

THE STELLAR UNIVERSE 

The stars appear fairly evenly distributed all around 
us, except in one portion of the sky where they 
seem very crowded, and so give one an impression 
of being very distant. This portion, known as the 
Milky Way, stretches, as we have already said, in 
the form of a broad band right round the entire 
heavens. In those regions of the sky most distant 
from the Milky Way the stars appear to be thinly 
sown, but become more and more closely massed 
together as the Milky Way is approached. 

This apparent distribution of the stars in space has 
given rise to a theory which was much favoured 
by Sir William Herschel, and which is usually 
credited to him, although it was really suggested by 
one Thomas Wright of Durham in 1750 ; that is to 
say, some thirty years or more before Herschel pro- 
pounded it. According to this, which is known as 
the " Disc " or *' Grindstone " Theory, the stars are 
considered as arranged in space somewhat in the 
form of a thick disc, or grindstone, close to the 
central parts of which our solar system is situated.^ 
Thus we should see a greater number of stars when 
we looked out through the length of such a disc in 

^ The Ptolemaic idea dies hard ! 



Astronomy of To-day 

any direction, than when we looked out through 
its breadth. This theory was, for a time, supposed 
to account quite reasonably for the Milky Way, 
and for the gradual increase in the number of stars 
in its vicinity. 

It is quite impossible to verify directly such a 
theory, for we know the actual distance of only about 
forty-three stars. We are unable, therefore, definitely 
to assure ourselves whether, as the grindstone theory 
presupposes, the stellar universe actually reaches 
out very much further from us in the direction of 
the Milky Way than in the other parts of the sky. 
The theory is clearly founded upon the supposition 
that the stars are more or less equal in size, and are 
scattered through space at fairly regular distances 
from each other. 

Brightness, therefore, had been taken as implying 
nearness to us, and faintness great distance. But 
we know to-day that this is not the case, and that 
the stars around us are, on the other hand, of various 
degrees of brightness and of all orders of size. Some 
of the faint stars — for instance, the galloping star 
in Pictor — are indeed nearer to us than many of 
the brighter ones. Sirius, c n the other hand, is twice 
as far off from us as a Centauri, and yet it is very 
much brighter ; while Canopus, which in brightness 
is second only to Sirius out of the whole sky, is too 
far off for its distance to be ascertained ! It must 
be remembered that no parallax had yet been found 
for any star in the days of Herschel, and so his 
estimations of stellar distances were necessarily of 
a very circumstantial kind. He did not, however, 
continue always to build upon such uncertain ground ; 

320 



The Stellar Universe 

but, after some further examination of the Milky 
Way, he gave up his idea that the stars were equally 
disposed in space, and eventually abandoned the 
grindstone theory. 

Since we have no means of satisfactorily testing the 
matter, through finding out the various distances from 
us at which the stars are really placed, one might 
just as well go to the other extreme, and assume that 
the thickening of stars in the region of the Milky 
Way is not an effect of perspective at all, but that 
the stars in that part of the sky are actually more 
crowded together than elsewhere — a thing which 
astronomers now believe to be the case. Looked at 
in this way, the shape of the stellar universe might be 
that of a globe-shaped aggregation of stars, in which 
the individuals are set at fairly regular distances from 
each other; the whole being closely encircled by a 
belt of densely packed stars. It must, however, be 
allowed that the gradual increase in the number of 
stars towards the Milky Way appears a strong argu- 
ment in favour of the grindstone theory ; yet the belt 
theory, as above detailed, seems to meet with more 
acceptance. 

There is, in fact, one marked circumstance which 
is remarkably difficult of explanation by means of the 
grindstone theory. This is the existence of vacant 
spaces — holes, so to speak, in the groundwork of the 
Milky Way. For instance, there is a cleft running 
for a good distance along its length, and there is 
also a starless gap in its southern portion. It seems 
rather improbable that such a great number of stars 
could have arranged themselves so conveniently, as 
to give us c. clear view right out into empty space 

321 X 



Astronomy of To-day 

through such a system in its greatest thickness ; as if, 
in fact, holes had been bored, and clefts made, from 
the boundary of the disc clean up to where our solar 
system lies. Sir John Herschel long ago drew atten- 
tion to this point very forcibly. It is plain that such 
vacant spaces can, on the other hand, be more simply 
explained as mere holes in a belt ; and the best 
authorities maintain that the appearance of the Milky 
Way confirms a view of this kind. 

Whichever theory be indeed the correct one, it 
appears at any rate that the stars do not stretch 
out in every direction to an infinite distance ; but 
that the stellar system is of limited extent^ and has in 
fact a boundary. 

In the first place, Science has no grounds for sup- 
posing that light is in any way absorbed or destroyed 
merely by its passage through the *^ ether," that 
imponderable medium which is beHeved to transmit 
the luminous radiations through space. This of course 
is tantamount to saying that all the direct light from 
all the stars should reach us, excepting that little 
which is absorbed in its passage through our own 
atmosphere. If stars, and stars, and stars existed 
in every direction outwards without end, it can be 
proved mathematically that in such circumstances 
there could not remain the tiniest space in the sky 
without a star to fill it, and that therefore the heavens 
would always blaze with light, and the night would 
be as bright as the noonday.^ How very far indeed 
this is from being the case, may be gathered from 
an estimate which has been made of the general 

* Even the Milky Way itself is far from being a blaze of light, which 
shows that the stars composing it do not extend outwards indefinitely. 

322 



The Stellar Universe 

amount of light which we receive from the stars. 
According to this estimate the sky is considered as 
more or less dark, the combined illumination sent 
to us by all the stars being only about the one- 
hundreth part of what we get from the full moon.^ 

Secondly, it has been suggested that although light 
may not suffer any extinction or diminution from 
the ether itself, still a great deal of illumination may 
be prevented from reaching us through myriads of 
extinguished suns, or dark meteoric matter lying 
about in space. The idea of such extinguished suns, 
dark stars in fact, seems however to be merely 
founded upon the sole instance of the invisible com- 
panion of Algol ; but, as we have seen, there is no 
proof whatever that it is a dark body. Again, some 
astronomers have thought that the dark holes in the 
Milky Way, ^^Coal Sacks," as they are called, are 
due to masses of cool, or partially cooled matter, 
which cuts off the light of the stars beyond. The 
most remarkable of these holes is one in the neigh- 
bourhood of the Southern Cross, known as the 
^^Coal Sack in Crux." But Mr. Gore thinks that 
the cause of the holes is to be sought for rather in 

1 Mr. Gore has recently made some remarkable deductions, with regard 
to the amount of light which we get from the stars. He considers that 
most of this light comes from stars below the sixth magnitude ; and con- 
sequently, if all the stars visible to the naked eye were to be blotted out, 
the glow of the night sky would remain practically the same as it is at 
present. Going to the other end of the scale, he thinks also that the 
combined light which we get from all the stars below the seventeenth 
magnitude is so very small, that it may be neglected in such an estimation. 
He finds, indeed, that if there are stars so low as the twentieth magnitude, 
one hundred millions of them would only be equal in brightness to a 
single first-magnitude star like Vega. On the other hand, it is possible 
that the light of the sky at night is not entirely due to starlight, but that 
some of it may be caused by phosphorescent glow. 



Astronomy of To-day 

what Sir William Herschel termed " clustering power/' 
i.e, 2L tendency on the part of stars to accumulate in 
certain places, thus leaving others vacant ; and the 
fact that globular and other clusters are to be found 
very near to such holes certainly seems corroborative 
of this theory. In summing up the whole question, 
Professor Newcomb maintains that there does not 
appear any evidence of the light from the Milky 
Way stars, which are apparently the furthest bodies 
we see, being intercepted by dark bodies or dark 
matter. As far as our telescopes can penetrate, he 
holds that we see the stars/^i"/ as they are. 

Also, if there did exist an infinite number of stars, 
one would expect to find evidence in some direction 
of an overpoweringly great force, — the centre of 
gravity of all these bodies. 

It is noticed, too, that although the stars increase 
in number with decrease in magnitude, so that as 
we descend in the scale we find three times as many 
stars in each magnitude as in the one immediately 
above it, yet this progression does not go on after 
a while. There is, in fact, a rapid falling off in 
numbers below the twelfth magnitude; which looks 
as if, at a certain distance from us, the stellar universe 
were beginning to thin out. 

Again, it is estimated, by Mr. Gore and others, 
that only about loo millions of stars are to be seen 
in the whole of the sky with the best optical aids. 
This shows well the limited extent of the stellar 
system, for the number is not really great. For 
instance, there are from fifteen to sixteen times as 
many persons alive upon the earth at this moment ! 

Last of all, there appears to be strong photographic 
324 



The Stellar Universe 

evidence that our sidereal system is limited in extent. 
Two photographs taken by the late Dr. Isaac Roberts 
of a region rich in stellar objects in the constellation 
of Cygnus, clearly show what has been so eloquently 
called the ^^ darkness behind the stars." One of these 
photographs was taken in 1895; and the other in 1898. 
On both occasions the state of the atmosphere was 
practically the same, and the sensitiveness of the films 
was of the same degree. The exposure in the first 
case was only one hour ; in the second it was about 
two hours and a half. And yet both photographs 
show exactly the same stars^ even down to the faintest. 
From this one would gather that the region in 
question, which is one of the most thickly star-strewn 
in the Milky Way, is penetrable right through with the 
means at our command. Dr. Roberts himself in 
commenting upon the matter drew attention to the 
fact, that many astronomers seemed to have tacitly 
adopted the assumption that the stars extend in- 
definitely through space. 

From considerations such as these the foremost 
astronomical authorities of our time consider them- 
selves justified in believing that the collection of stars 
around us is finite ; and that although our best tele- 
scopes may not yet be powerful enough to penetrate 
to the final stars, still the rapid decrease in numbers 
as space is sounded with increasing telescopic power, 
points strongly to the conclusion that the boundaries 
of the stellar system may not lie very far beyond the 
uttermost to which we can at present see. 

Is it possible then to make an estimate of the extent 
of this stellar system ? 

Whatever estimates we may attempt to form cannot 
325 



Astronomy of To-day 

however be regarded as at all exact, for we know the 
actual distances of such a very few only of the nearest 
of the stars. But our knowledge of the distances even 
of these few, permits us to assume that the stars close 
around us may be situated, on an average, at about 
eight light-years from each other ; and that this holds 
good of the stellar spaces, with the exception of the 
encircling girdle of the Milky Way, where the stars 
seem, actually to be more closely packed together. 
This girdle further appears to contain the greater 
number of the stars. Arguing along these lines, 
Professor Newcomb reaches the conclusion that the 
farthest stellar bodies which we see are situated at 
about between 3000 and 4000 light-years from us. 

Starting our inquiry from another direction, we 
can try to form an estimate by considering the ques- 
tion of proper motions. 

It will be noticed that such motions do not depend 
entirely upon the actual speed of the stars themselves, 
but that some of the apparent movement arises in- 
directly from the speed of our own sun. The part 
in a proper motion which can be ascribed to the 
movement of our solar system through space is clearly 
a displacement in the nature of a parallax — Sir William 
Herschel called it ^* Systematic Parallax " ; so that know- 
ing the distance which we move over in a certain lapse 
of time, we are able to hazard a guess at the distances 
of a good many of the stars. An inquiry upon such 
lines must needs be very rough, and is plainly based 
upon the assumption that the stars whose distances 
we attempt to estimate are moving at an average 
speed much like that of our own sun, and that they 
are not ^^ runaway stars" of the 1830 Groombridge 

326 



The Stellar Universe 

order. Be that as it may, the results arrived at by 
Professor Newcomb from this method of reasoning 
are curiously enough very much on a par with those 
founded on the few parallaxes which we are really 
certain about ; with the exception that they point to 
somewhat closer intervals between the individual stars, 
and so tend to narrow down our previous estimate of 
the extent of the stellar system. 

Thus far we get, and no farther. Our solar system 
appears to lie somewhere near the centre of a great 
collection of stars, separated each one from the other, 
on an average, by some 40 bilhons of miles ; the whole 
being arranged in the form of a mighty globular 
cluster. Light from the nearest of these stars takes 
some four years to come to us. It takes about 1000 
times as long to reach us from the confines of the 
system. This globe of stars is wrapt around closely 
by a stellar girdle, the individual stars in which are 
set together more densely than those in the globe 
itself. The entire arrangement appears to be con- 
structed upon a very regular plan. Here and there, 
as Professor Newcomb points out, the aspect of the 
heavens differs in small detail ; but generally it may be 
laid down that the opposite portions of the sky, whether 
in the Milky Way itself, or in those regions distant 
from it, show a marked degree of symmetry. The 
proper motions of stars in corresponding portions of 
the sky reveal the same kind of harmony, a harmony 
which may even be extended to the various colours of 
the stars. The stellar system, which we see disposed 
all around us, appears in fine to bear all the marks of 
an organised whole. 

The older astronomers, to take Sir William Herschel 

327 



Astronomy of To-day 

as an example, supposed some of the nebulae to be 
distant ^' universes." Sir William was led to this con- 
clusion by the idea he had formed that, when his 
telescopes failed to show the separate stars of which 
he imagined these objects to be composed, he must 
put down the failure to their stupendous distance 
from us. For instance, he thought the Orion Nebula, 
which is now known to be made up of glowing gas, 
to be an external stellar system. Later on, however, 
he changed his mind upon this point, and came to the 
conclusion that ^' shining fluid " would better account 
both for this nebula, and for others which his tele- 
scopes had failed to separate into component stars. 

The old ideas with regard to external systems and 
distant universes have been shelved as a consequence 
of recent research. All known clusters and nebulae 
are now firmly believed to lie within our stellar 
system. 

This view of the universe of stars as a sort of island 
in the immensities, does not, however, give us the 
least idea about the actual extent of space itself. 
Whether what is called space is really infinite, that is 
to say, stretches out unendingly in every direction, or 
whether it has eventually a boundary somewhere, are 
alike questions which the human mind seems utterly 
unable to picture to itself. 



328 



CHAPTER XXVI 

THE STELLAR \Jl<llY ERSE— continued 

It is very interesting to consider the proper motions 
of stars with reference to such an isolated stellar 
system as has been pictured in the previous chapter. 
These proper motions are so minute as a rule, that we 
are quite unable to determine whether the stars which 
show them are moving along in straight lines, or in 
orbits of immense extent. It would, in fact, take 
thousands of years of careful observation to determine 
whether the paths in question showed any degree of 
curving. In the case of the more distant stars, the 
accurate observations which have been conducted 
during the last hundred years have not so far revealed 
any proper motions with regard to them ; but one 
cannot escape the conclusion that these stars move as 
the others do. 

If space outside our stellar system is infinite in 
extent, and if all the stars within that system are 
moving unchecked in every conceivable direction, the 
result must happen that after immense ages these 
stars will have drawn apart to such a distance from 
each other, that the system will have entirely disin- 
tegrated, and will cease to exist as a connected whole. 
Eventually, indeed, as Professor Newcomb points out, 
the stars will have separated so far from each other 

329 



Astronomy of To-day 

that each will be left by itself in the midst of a black 
and starless sky. If, however, a certain proportion of 
stars have a speed sufficiently slow, they will tend 
under mutual attraction to be brought to rest by 
collisions, or forced to move in orbits around each 
other. But those stars which move at excessive 
speeds, such, for instance, as 1830 Groombridge, or 
the star in the southern constellation of Pictor, seem 
utterly incapable of being held back in their courses 
by even the entire gravitative force of our stellar 
system acting as a whole. These stars must, there- 
fore, move eventually right through the system and 
pass out again into the empty spaces beyond. Add 
to this ; certain investigations, made into the speed of 
1830 Groombridge, furnish a remarkable result. It is 
calculated, indeed, that had this star been falling 
through infinite space for ever^ pulled towards us by 
the combined gravitative force of our entire system of 
stars, it could not have gathered up anything like the 
speed with which it is at present moving. No force, 
therefore, which we can conjure out of our visible 
universe, seems powerful enough either to have im- 
pressed upon this runaway star the motion which it 
now has, nor to stay it in its wild course. What an 
astounding condition of things ! 

Speculations like this call up a suspicion that there 
may yet exist other universes, other centres of force, 
notwithstanding the apparent solitude of our stellar 
system in space. It will be recollected that the idea 
of this isolation is founded upon such facts as, that 
the heavens do not blaze with light, and that the 
stars gradually appear to thin out as we penetrate 
the system with increasing telescopic power. But 

330 



The Stellar Universe 

perchance there is something which hinders us from 
seeing out into space beyond our cluster of stars ; 
which prevents light, in fact, from reaching us from 
other possible systems scattered through the depths 
beyond. It has, indeed, been suggested by Mr. Gore ^ 
that the light-transmitting ether may be after all 
merely a kind of ^' atmosphere " of the stars ; and 
that it may, therefore, thin off and cease a little be- 
yond the confines of our stellar system, just as the 
air thins off and practically ceases at a comparatively 
short distance from the earth. A clashing together 
of solid bodies outside our atmosphere could plainly 
send us no sound, for there is no air extending the 
whole way to bear to our ears the vibrations thus 
set up ; so light emitted from any body lying beyond 
our system of stars, would not be able to come to 
us if the ether, whose function it is to convey the 
rays of light, ceased at or near the confines of that 
system. 

Perchance we have in this suggestion the key to 
the mystery of how our sun and the other stellar 
bodies maintain their functions of temperature and 
illumination. The radiations of heat and light arriv- 
ing at the limits of this ether, and unable to pass any 
further, may be thrown back again into the system 
in some altered form of energy. 

But these, at best, are mere airy and fascinating 
speculations. We have, indeed, no evidence whatever 
that the luminiferous ether ceases at the boundary of 
the stellar system. If, therefore, it extends outwards 
infinitely in every direction, and if it has no absorb- 

^ Planetary and Stellar Studies y by John Ellard Gore, F.R.A.S., 
M.R.I.A., London, 1888. 



Astronomy of To-day 

ing or weakening effect on the vibrations which it 
transmits, we cannot escape from the conclusion 
that practically all the rays of light ever emitted by 
all the stars must chase one another eternally through 
the never-ending abysses of space. 



332 



CHAPTER XXVII 

THE BEGINNING OF THINGS 

Laplace's Nebular Hypothesis 

Dwelling upon the fact that all the motions of 
revolution and rotation in the solar system, as known 
in his day, took place in the same direction and 
nearly in the same plane, the great French astronomer, 
Laplace, about the year 1796, put forward a theory 
to account for the origin and evolution of that system. 
He conceived that it had come into being as a result 
of the gradual contraction, through cooling, of an 
intensely heated gaseous lens-shaped mass, which 
had originally occupied its place, and had extended 
outwards beyond the orbit of the furthest planet. 
He did not, however, attempt to explain how such a 
mass might have originated ! He went on to suppose 
that this mass, in soine manner, perhaps by mutual 
gravitation among its parts, had acquired a motion 
of rotation in the same direction as the planets now 
revolve. As this nebulous mass parted with its heat 
by radiation, it contracted towards the centre. Be- 
coming smaller and smaller, it was obliged to rotate 
faster and faster in order to preserve its equilibrium. 
Meanwhile, in the course of contraction, rings of 
matter became separated from the nucleus of the 
mass, and were left behind at various intervals. These 
rings were swept up into subordinate masses similar 

333 



Astronomy of To-day 

to the original nebula. These subordinate masses also 
contracted in the same manner, leaving rings behind 
them which; in turn, were swept up to form satellites. 
Saturn's ring was considered, by Laplace, as the only 
portion of the system left which still showed traces 
of this evolutionary process. It is even probable that 
it may have suggested the whole of the idea to him. 

Laplace was, however, not the first philosopher who 
had speculated along these lines concerning the origin 
of the world. 

Nearly fifty years before, in 1750 to be exact, 
Thomas Wright, of Durham, had put forward a theory 
to account for the origin of the whole sidereal universe. 
In his theory, however, the birth of our solar system 
was treated merely as an incident. Shortly afterwards 
the subject was taken up by the famous German 
philosopher, Kant, who dealt with the question in 
a still more ambitious manner, and endeavoured to 
account in detail for the origin of the solar system as 
well as of the sidereal universe. Something of the 
trend of such theories may be gathered from the 
remarkable lines in Tennyson's Princess : — 

" This world was once a fluid haze of light, 
Till toward the centre set the starry tides, 
And eddied into suns, that wheeHng cast 
The planets." 

The theory, as worked out by Kant, was, however, 
at the best merely a tour de force of philosophy. 
Laplace's conception was much less ambitious, for 
it did not attempt to explain the origin of the entire 
universe, but only of the solar system. Being thus 
reasonably Hmited in its scope, it more easily obtained 
credence. The arguments of Laplace were further 

334 



The Beginning of Things 

founded upon a mathematical basis. The great place 
which he occupied among the astronomers of that 
time caused his theory to exert a preponderating 
influence on scientific thought during the century 
which followed. 

A modification of Laplace's theory is the Meteo- 
ritic Hypothesis of Sir Norman Lockyer. Accord- 
ing to the views of that astronomer, the material of 
which the original nebula was composed is presumed 
to have been in the meteoric, rather than in the 
gaseous, state. Sir Norman Lockyer holds, indeed, 
that nebulae are, in reality, vast swarms of meteors, 
and the Hght they emit results from continual colli- 
sions between the constituent particles. The French 
astronomer, Faye, also proposed to modify Laplace's 
theory by assuming that the nebula broke up into 
rings all at once, and not in detail, as Laplace had 
wished to suppose. 

The hypothesis of Laplace fits in remarkably well 
with the theory put forward in later times by Helm- 
holtz, that the heat of the sun is kept up by the 
continual contraction of its mass. It could thus have 
only contracted to its present size from one very 
much larger. 

Plausible, however, as Laplace's great hypothesis 
appears on the surface, closer examination shows 
several vital objections, a few of those set forth by 
Professor Moulton being here enumerated — 

Although Laplace held that the orbits of the 
planets were sufficiently near to being in the one 
plane to support his views, yet later investigators 
consider that their very deviations from this plane 
are a strong argument against the hypothesis. 

335 



Astronomy of To-day 

Again, it is thought that if the theory were the 
correct explanation, the various orbits of the planets 
would be much more nearly circular than they are. 

It is also thought that such interlaced paths, as 
those in which the asteroids and the little planet 
Eros move, are most unlikely to have been pro- 
duced as a result of Laplace's nebula. 

Further, while each of the rings was sweeping 
up its matter into a body of respectable dimen- 
sions, its gravitative power would have been for the 
time being so weak, through being thus spread out, 
that any lighter elements, as, for instance, those of 
the gaseous order, would have escaped into space 
in accordance with the principles of the kinetic 
theory. 

The idea that rings would at all be left behind at 
certain intervals during the contraction of the nebula 
iSf perhaps, one of the weakest points in Laplace's hypo- 
thesis. 

Mathematical investigation does not go to show 
that the rings, presuming they could be left behind 
during the contraction of the mass, would have 
aggregated into planetary bodies. Indeed, it rather 
points to the reverse. 

Lastly, such a discovery as that the ninth satellite 
of Saturn revolves in a retrograde direction— that is 
to say, in a direction contrary to the other revolu- 
tions and rotations in our solar system — appears 
directly to contradict the hypothesis. 

Although Laplace's hypothesis seems to break 
down under the keen criticism to which it has been 
subjected, yet astronomers have not relinquished the 
idea that our solar system has probably had its 

336 



The Beginning of Things 

origin from a nebulous mass. But the apparent 
failure of the Laplacian theory is emphasised by the 
fact, that not a single example of a nebula, in the course 
of breaking up into concentric rings, is known to exist 
in the entire heaven. Indeed, as we saw in Chapter 
XXIV., there seems to be no reliable example of 
even a ''ring" nebula at all. Mr. Gore has pointed 
this out very succinctly in his recently published 
work. Astronomical Essays, where he says: — ''To 
any one who still persists in maintaining the hypo- 
thesis of ring formation in nebulae, it may be said 
that the whole heavens are against him." 

The conclusions of Keeler already alluded to, that 
the spiral is the normal type of nebula, has led during 
the past few years to a new theory by the American 
astronomers. Professors Chamberlin and Moulton. 
In the detailed account of it which they have set forth, 
they show that those anomahes which were stumbling- 
blocks to Laplace's theory do not contradict theirs. 
To deal at length with this theory, to which the name 
of " Planetesimal Hypothesis " has been given, would 
not be possible in a book of this kind. But it may 
be of interest to mention that the authors of the 
theory in question remount the stream of time still 
further than did Laplace, and seek to explain the origin 
of the spiral nebulae themselves in the following 
manner : — 

Having begun by assuming that the stars are 
moving apparently in every direction with great 
velocities, they proceed to point out that sooner or 
later, although the lapse of time may be extraordinarily 
long, collisions or near approaches between stars 
are bound to occur. In the case of collisions the 

-337 Y 



Astronomy of To-day 

chances are against the bodies striking together 
centrally^ it being very much more likely that they 
will hit each other rather towards the side. The 
nebulous mass formed as a result of the disintegration 
of the bodies through their furious impact would 
thus come into being with a spinning movement, 
and a spiral would ensue. Again, the stars may not 
actually collide, but merely approach near to each 
other. If very close, the interaction of gravitation 
will give rise to intense strains, or tides, which will 
entirely disintegrate the bodies, and a spiral nebula 
will similarly result. As happens upon our earth, 
two such tides would rise opposite to each other ; 
and, consequently, it is a noticeable fact that spiral 
nebulae have almost invariably two opposite branches 
(see Plate XXII., p 314). Even if not so close, the 
gravitational strains set up would produce tremendous 
eruptions of matter ; and in this case, a spiral move- 
ment would also be generated. On such an assump- 
tion the various bodies of the solar system may be 
regarded as having been ejected from parent masses. 

The acceptance of the Planetesimal Hypothesis in 
the place of the Hypothesis of Laplace will not, as 
we have seen, by any means do av/ay with the proba- 
bility that our solar system, and similar systems, 
have originated from a nebulous mass. On the 
contrary it puts that idea on a firmer footing than 
before. The spiral nebulae which we see in the 
heavens are on a vast scale, and may represent the 
formation of stellar systems and globular clusters. 
Our solar system may have arisen from a small 
spiral. 

We will close these speculations concerning the 

338 



The Beginning of Things 

origin of things with a short sketch of certain in- 
vestigations made in recent years by Sir George H. 
Darwin, of Cambridge University, into the question 
of the probable birth of our moon. He comes to the 
conclusion that at least fifty-four millions of years ago 
the earth and moon formed one body, which had a 
diameter of a little over 8000 miles. This body 
rotated on an axis in about five hours, namely, about 
five times as fast as it does at present. The rapidity 
of the rotation caused such a tremendous strain that 
the mass was in a condition of, what is called, un- 
stable equilibrium ; very little more, in fact, being 
required to rend it asunder. The gravitational pull of 
the sun, which, as we have already seen, is in part the 
cause of our ordinary tides, supplied this extra strain, 
and a portion of the mass consequently broke off, 
which receded gradually from the rest and became 
what we now know as the moon. Sir George Darwin 
holds that the gravitational action of the sun will in 
time succeed in also disturbing the present apparent 
harmony of the earth-moon system, and will eventu- 
ally bring the moon back towards the earth, so that 
after the lapse of great ages they will re-unite once 
again. 

In support of this theory of the terrestrial origin 
of the moon, Professor W. H. Pickering has put 
forward a bold hypothesis that our satellite had its 
origin in the great basin of the Pacific. This ocean 
is roughly circular, and contains no large land 
masses, except the Australian Continent. He sup- 
poses that, prior to the moon's birth, our globe was 
already covered with a slight crust. In the tearing 
away of that portion which was afterwards destined 

339 



Astronomy of To-day 

to become the moon the remaining area of the crust 
was rent in twain by the shock ; and thus were 
formed the two great continental masses of the Old 
and New Worlds. These masses floated apart across 
the fiery ocean, and at last settled in the positions 
which they now occupy. In this way Professor 
Pickering explains the remarkable parallelism which 
exists between the opposite shores of the Atlantic. The 
fact of this parallelism had, however, been noticed 
before ; as, for example, by the late Rev. S. ]. John- 
son, in his book Eclipses^ Past and Future^ where we 
find the following passage : — 

^^ If we look at our maps we shall see the parts 
of one Continent that jut out agree with the indented 
portions of another. The prominent coast of Africa 
would fit in the opposite opening between North 
and South America, and so in numerous other in- 
stances. A general rending asunder of the World 
would seem to have taken place when the foundations 
of the great deep were broken up." 

Although Professor Pickering's theory is to a 
certain degree anticipated in the above words, still 
he has worked out the idea much more fully, and 
given it an additional fascination by connecting it 
with the birth of the moon. He points out, in fact, 
that there is a remarkable similarity between the 
lunar volcanoes and those in the immediate , neigh- 
bourhood of the Pacific Ocean. He goes even further 
to suggest that Australia is another portion of the 
primal crust which was detached out of the region 
now occupied by the Indian Ocean, where it was 
originally connected with the south of India or the 
east of Africa. 

340 



The Beginning of Things 

Certain objections to the theory have been put 
forward, one of which is that the paralleHsm noticed 
between the opposite shores of the Atlantic is almost 
too perfect to have remained through some sixty 
millions of years down to our own day, in the face 
of all those geological movements of upheaval and 
submergence, which are perpetually at work upon our 
globe. Professor Pickering, however, replies to this 
objection by stating that many geologists believe 
that the main divisions of land and water on the 
earth are permanent, and that the geological altera- 
tions which have taken place since these were formed 
have been merely of a temporary and superficial 
nature. 



341 



CHAPTER XXVIII 

THE END OF THINGS 

We have been trying to picture the beginning of 
things. We will now try to picture the end. 

In attempting this, we find that our theories must 
of necessity be limited to the earth, or at most to the 
solar system. The time-honoured expression ^^End 
of the World " really applies to very little beyond the 
end of our own earth. To the people of past ages it, 
of course, meant very much more. For them, as we 
have seen, the earth was the centre of everything ; 
and the heavens and all around were merely a kind 
of minor accompaniment, created, as they no doubt 
thought, for their especial benefit. In the ancient 
view, therefore, the beginning of the earth meant the 
beginning of the universe, and the end of the earth 
the extinction of all things. The belief, too, was 
general that this end would be accomplished through 
fire. In the modern view, however, the birth and 
death of the earth, or indeed of the solar system, 
might pass as incidents almost unnoticed in space. 
They would be but mere links in the chain of cosmic 
happenings. 

A number of theories have been forward from time 
to time prognosticating the end of the earth, and 
consequently of human life. We will conclude with 

342 



The End of Things 

a recital of a few of them, though which, if any, is 
the true one, the Last Men alone can know. 

Just as a living creature may at any moment die in 
the fulness of strength through sudden malady or 
accident, or, on the other hand, may meet with death 
as a mere consequence of old age, so may our globe 
be destroyed by some sudden cataclysm, or end in 
slow processes of decay. Barring accidents, there- 
fore, it would seem probable that the growing cold of 
the earth, or the gradual extinction of the sun, should 
after many millions of years close the chapter of life, 
as we know it. On the former of these supposi- 
tions, the decrease of temperature on our globe might 
perhaps be accelerated by the thinning of the atmos- 
phere, through the slow escape into space of its 
constituent gases, or their gradual chemical combina- 
tion with the materials of the earth. The subterranean 
heat entirely radiated away, there would no longer 
remain any of those volcanic elevating forces which 
so far have counteracted the slow wearing down of 
the land surface of our planet, and thus what water 
remained would in time wash over all. If this pre- 
ceded the growing cold of the sun, certain strange 
evolutions of marine forms of life would be the last 
to endure, but these, too, would have to go in the 
end. 

Should, however, the actual process be the reverse 
of this, and the sun cool down the quicker, then man 
would, as a consequence of his scientific knowledge, 
tend in all probability to outlive the other forms of 
terrestrial life. In such a vista we can picture the 
regions of the earth towards the north and south 
becoming gradually more and more uninhabitable 

343 



Astronomy of To-day 

through cold, and human beings withdrawing before 
the slow march of the icy boundary, until the only 
regions capable of habitation would lie within the 
tropics. In such a struggle between man and destiny 
science would be pressed to the uttermost, in the 
devising of means to counteract the slow diminution 
of the solar heat and the gradual disappearance of 
air and water. By that time the axial rotation of our 
globe might possibly have been slowed down to such 
an extent that one side alone of its surface would be 
turned ever towards the fast dying sun. And the 
mind's eye can picture the last survivors of the human 
race, huddled together for warmth in a glasshouse 
somewhere on the equator, waiting for the end to 
come. 

The mere idea of the decay and death of the solar 
system almost brings to one a cold shudder. All that 
sun's light and heat, which means so much to us, 
entirely a thing of the past. A dark, cold ball rushing 
along in space, accompanied by several dark, cold 
balls circling ceaselessly around it. One of these a 
mere cemetery, in which there would be no longer 
any recollection of the mighty empires, the loves and 
hates, and all that teeming play of life which we call 
History. Tombstones of men and of deeds, whirling 
along forgotten in the darkness and silence. Sic 
transit gloria mundi. 

In that brilliant flight of scientific fancy, the Time 
Machine^ Mr. H. G. Wells has pictured the closing 
years of the earth in some such long-drawn agony as 
this. He has given us a vision of a desolate beach 
by a salt and almost motionless sea. Foul monsters 
of crab-Hke form crawl slowly about, beneath a huge 

344 



The End of Things 

hull of sun, red and fixed in the sky. The rocks 
around are partly coated with an intensely green 
vegetation, like the lichen in caves, or the plants 
which grow in a perpetual twilight. And the air 
is now of an exceeding thinness. 

He dips still further into the future, and thus pre- 
dicts the final form of life : — 

" I saw again the moving thing upon the shoal — 
there was no mistake now that it was a moving thing 
— against the red water of the sea. It was a round 
thing, the size of a football perhaps, or it may be 
bigger, and tentacles trailed down from it ; it seemed 
black against the weltering blood-red water, and it 
was hopping fitfully about." 

What a description of the ^' Heir of all the Ages 1 " 

To picture the end of our world as the result of a 
cataclysm of some kind, is, on the other hand, a form 
of speculation as intensely dramatic as that with which 
we have just been dealing is unutterably sad. 

It is not so many years ago, for instance, that men 
feared a sudden catastrophe from the possible collision 
of a comet with our earth. The unreasoning terror 
with which the ancients were wont to regard these 
mysterious visitants to our skies had, indeed, been 
replaced by an apprehension of quite another kind. 
For instance, as we have seen, the announcement 
in 1832 that Biela's Comet, then visible, would cut 
through the orbit of the earth on a certain date threw 
many persons into a veritable panic. They did not 
stop to find out the real facts of the case, namely, that, 
at the time mentioned, the earth would be nearly a 
month's journey from the point indicated ! 

It is, indeed, very difficult to say what form of 
345 



Astronomy of To-day 

damage the earth would suffer from such a colUsion. 
In 1861 it passed, as we have seen, through the tai^ 
of the comet without any noticeable result. But the 
head of a comet, on the other hand, may, for aught 
we know, contain within it elements of peril for us. 
A colHsion with this part might, for instance, result 
in a violent bombardment of meteors. But these 
meteors could not be bodies of any great size, for the 
masses of comets are so very minute that one can 
hardly suppose them to contain any large or dense 
constituent portions. 

The danger, however, from a comet's head might 
after all be a danger to our atmosphere. It might 
precipitate, into the air, gases which would asphyxiate 
us or cause a general conflagration. It is scarcely 
necessary to point out that dire results would fol- 
low upon any interference with the balance of our 
atmosphere. For instance, the well-known French 
astronomer, M. Camille Flammarion,^ has imagined 
the absorption of the nitrogen of the air in this way ; 
and has gone on to picture men and animals reduced 
to breathing only oxygen, first becoming excited, then 
mad, and finally ending in a perfect saturnalia of 
delirium. 

Lastly, though we have no proof that stars event- 
ually become dark and cold, for human time has so 
far been all too short to give us even the smallest 
evidence as to whether heat and light are diminishing 
in our own sun, yet it seems natural to suppose that 
such bodies must at last cease their functions, like 

^ See his work, La Fin dii Monde, wherein the various ways by 
which our world may come to an end are dealt with at length, and in a 
profoundly interesting manner. 



The End of Things 

everything else which we know of. We may, there- 
fore, reasonably presume that there are dark bodies 
scattered in the depths of space. We have, indeed, a 
suspicion of at least one, though perhaps it partakes 
rather of a planetary nature, namely, that ^Mark" 
body which continually eclipses Algol, and so causes 
the temporary diminution of its light. As the sun 
rushes towards the constellation of Lyra such an extin- 
guished sun may chance to find itself in his path ; just 
as a derelict hulk may loom up out of the darkness right 
beneath the bows of a vessel sailing the great ocean. 

Unfortunately a collision between the sun and a 
body of this kind could not occur with such merciful 
suddenness. A tedious warning of its approach would 
be given from that region of the heavens whither our 
system is known to be tending. As the dark object 
would become visible only when sufficiently near our 
sun to be in some degree illuminated by his rays, it 
might run the chance at first of being mistaken for a 
new planet. If such a body were as large, for instance, 
as our own sun, it should, according to Mr. Gore's 
calculations, reveal itself to the telescope some fifteen 
years before the great catastrophe. Steadily its disc 
would appear to enlarge, so that, about nine years 
after its discovery, it would become visible to the 
naked eye. At length the doomed inhabitants of the 
earth, paralysed with terror, would see their relentless 
enemy shining Hke a second moon in the northern 
skies. Rapidly increasing in apparent size, as the 
gravitational attractions of the solar orb and of itself 
interacted more powerfully with diminishing distance, 
it would at last draw quickly in towards the sun and 
disappear in the glare. 

347 



Astronomy of To-day 

It is impossible for us to conceive anything more 
terrible than these closing days, for no menace of 
catastrophe which we can picture could bear within 
it such a certainty of fulfilment. It appears, therefore, 
useless to speculate on the probable actions of men in 
their now terrestrial prison. Hope, which so far had 
buoyed them up in the direst calamities, would here 
have no place. Humanity, in the fulness of its 
strength, would await a wholesale execution from 
which there could be no chance at all of a reprieve. 
Observations of the approaching body would have 
enabled astronomers to calculate its path with great 
exactness, and to predict the instant and character of 
the impact. Eight minutes after the moment allotted 
for the collision the resulting tide of flame would 
surge across the earth's orbit, and our globe would 
quickly pass away in vapour. 

And what then ? 

A nebula, no doubt ; and after untold ages the 
formation possibly from it of a new system, rising 
phoenix-like from the vast crematorium and filling 
the place of the old one. A new central sun, perhaps, 
with its attendant retinue of planets and satellites. 
And teeming life, perchance, appearing once more in 
the fulness of time, when temperature in one or other 
of these bodies had fallen within certain limits, and 
other predisposing conditions had supervened. 

" The world's great age begins anew, 

The golden years return, 

The earth doth like a snake renew 

Her winter weeds outworn : 
Heaven smiles, and faiths and empires gleam 
Like wrecks of a dissolving dream. 

348 



The End of Things 

A brighter Hellas rears its mountains 

From waves serener far ; 
A new Peneus rolls his fountains 

Against the morning star ; 
Where fairer Tempes bloom, there sleep 
Young Cyclads on a sunnier deep. 

A loftier Argo cleaves the main, 
Fraught with a later prize ; 

Another Orpheus sings again, 

And loves, and weeps, and dies ; 

A new Ulysses leaves once more 

Calypso for his native shore. 



Oh cease ! must hate and death return ? 

Cease ! must men kill and die ? 
Cease ! drain not to its dregs the urn 

Of bitter prophecy ! 
The world is weary of the past, — 
Oh might it die or rest at last ! " 



349 



INDEX 



Achromatic telescope, 115, 116 

Adams, 24, 236, 243 

Aerial telescopes, no, in 

Agathocles, Eclipse of, 85 

Agrippa, Camillus, 44 

Ahaz, dial of, 85 

Air, 166 

Airy, Sir G. B., 92 

Al gul, 307 

Al Sufi, 284, 290, 296, 315 

Alcor, 294 

Alcyone, 284 

Aldebaran, 103, 288, 290, 297 

Algol, 307, 309-310, 312, 323, 347 

Alpha, Centauri, 52-53, 280, 298- 

299. 304, 320 
Alpha Crucis, 298 
Alps, Lunar, 200 
Altair, 295 

Altitude of objects in sky, 196 
Aluminium, 145 
Amos viii. 9, 85 
Anderson, T. D., 311-312 
Andromeda (constellation), 279, 

314; Great Nebula in, 314, 316 
Andromedid meteors, 272 
Anglo-Saxon Chronicle, 87-88 
Anighito meteorite, 277 
Annular eclipse, 65-68, 80, 92, 99 
Annular Nebula in Lyra, 315-316 
Annulus, 68 
Ansae, 242-243 

Anticipation in discovery, 236-237 
Apennines, Lunar, 200 
Aphelion, 274 
Apparent enlargement of celestial 

objects, 192-196 
Apparent size of celestial objects 

deceptive, 196, 294 
Apparent sizes of sun and moon, 

variations in, 6"]^ 80, 178 



Aquila (constellation), 295 

Arabian astronomers, 107, 307 

Arago, 92, 257 

Arc, degrees minutes and seconds 
of, 60 

Arcturus, 280, 282, 290, 295 

Argelander, 290 

Argo (constellation), 298 

Aristarchus of Samos, 171 

Aristarchus (lunar crater), 205 

Aristophanes, loi 

Aristotle, 161, 173, 185 

Arrhenius 222, 253-254 

Assyrian tablet, 84 

Asteroidal zone, analogy of, to 
Saturn's rings, 238 

Asteroids (or minor planets), 30-31, 
225-228, 336; discovery of the, 
23, 244; Wolf's method of dis- 
covering, 226-227 

Astrology, 56 

Astronomical Essays, 63, 337 

Astronomical Society, Royal, 144 

Astronomy, Manual oi, 166 

Atlantic Ocean, parallelism of 
opposite shores, 34O-341 

Atlas, the Titan, 18 

Atmosphere, absorption by earth's, 
129-130; ascertainment of, by 
spectroscope, 124-125, 212; 
height of earth's, 167, 267 ; of 
asteroids, 226; of earth, 129, 130, 
166-169, 218, 222, 267, 346; of 
Mars, 156,212,216; of Mercury, 
156; of moon, 70-71, 156, 201- 
203; of Jupiter, 231 ; of planets, 
125 ; of Saturn's rings, 239 

" Atmosphere " of the stars, 331 

Atmospheric layer and " glass- 
house " compared, 167, 203 

August Meteors (Perseids), 270 



35- 



Index 



Auriga (constellation), 294-296, 306, 

311 ; New Star in, 311 
Aurigae, /3 (Beta), 294, 297, 304 
Aurora Borealis, 141, 143, 259 
Australia, suggested origin of, 340 
Axis, 29-30; of earth, 163, 180; 

small movement of earth's, 180- 

181 

Babylonian tablet, 84 

Babylonian idea of the moon, 185 

Bacon, Roger, 108 

Bacubirito meteorite, 277 

Bagdad, 107 

Baily, Francis, 92 

" Baily's Beads," 69, 70, 91-92, 154 

Bailly (lunar crater), 199 

Ball, Sir Robert, 271 

Barnard, E. E., 31, 224, 232-234, 

237, 258 
" Bay of Rainbows," 197 
Bayer's classification of stars, 289, 

291-292 
Bayeux Tapestry, 263 
Bear, Great (constellation). ^'^^Ursa 

Major ; Little, see Ursa Minor 
Beehive (Praesepe), 307 
Beer, 206 
Belopolsky, 304 
"Belt" of Orion, 297 
Belt theory of Milky Way, 321 
Belts of Jupiter, 230 
Bergstrand, 314 
Berlin star chart, 244 
Bessel, 173, 280, 305 
Beta (iS) Lyrae, 307 
Beta (/3) Persei. See Algol 
Betelgeux, 297 
Bible, ecHpses in, 85 
Biela's Comet, 256-257, 272-273, 

345 
Bielids, 270, 272-273 
Billion, 51-52 
Binary stars, spectroscopic, 301-306, 

309 ; visual, 300, 303-306 
" Black Drop," 152-154 
"Black Hour," 89 
"Black Saturday," 89 
Blood, moon in eclipse like, 102 
Blue (rays of light), 121, 130 
Bode's Law, 22-23, 244-245 
Bolometer, 127 



Bond, G. P., 236, 257 

Bonpland, 270 

Bootes (constellation), 295, 314 

Bradley, iii 

Brahe, Tycho, 290, 311 

Bredikhine's theory of comets' tails, 
253-254, 256 

Bright eclipses of moon, 65, 102 

British Association for the Advance- 
ment of Science, 318 

British Astronomical Association, 
Journal of , 194 

British Museum, 84 

Bull (constellation). See Taurus ; 
" Eye " of the, 297 ; " Head " of 
the, 297 

Burgos, 98 

Busch, 93 

C^SAR, Julius, 85, no, iSo, 259, 

262, 291, 293 
Calcium, 138, 145 
Callisto, 233-234 
Cambridge, 24, 91, 119, 243 
Campbell, 305 
Canali, 214 
" Canals" of Mars, 214-222, 224- 

225 
Cancer (constellation), 307 
Canes Venatici (constellation), 306, 

314 
Canis Major (constellation), 289, 

296-297 ; Minor, 296-297 
Canopus, 285, 298-299, 320 
Capella, 280, 282, 290, 294, 297, 

303* 313 

Carbon, 145 

Carbon dioxide. See Carbonic acid 
gas 

Carbonic acid gas, 166, 213, 221-222 

Carnegie Institution, Solar Obser- 
vatory of, 118 

Cassegrainian telescope, 114, iiS 

Cassini, J. D., 236, 240 

" Cassini's Division" in Saturn's 
ring, 236, 238 

Cassiopeia (constellation), 279, 294, 

311, 314 
Cassiopeise, 9j(Eta), 303 
Cassiopeia's Chair, 294 
Cassius, Dion, 86 
Castor, 282, 297, 304 



352 



Index 



Catalogues of stars, 1 06, 290-291, 31 1 
Centaur. See Centaurus 
Centaurus (constellation), 298, 306 
Centre of gravity, 42, 283-284, 324 
Ceres, diameter of, 30, 225 
Ceti, Omicron (or Mira), 307-308 
Cetus, or the Whale (constellation), 

307 

Chaldean astronomers, 74, 76 

Challis, 243-244 

Chamberlin, 337 

" Chambers of the South," 299 

Chandler, 308 

Charles V., 261 

"Charles' Wain," 291 

Chemical rays, 127 

Chinese and eclipses, 83 

Chloride of sodium, 122 

Chlorine, 122, 145 

Christ, Birth of, 102 

Christian Era, first recorded solar 
eclipse in, 85 

Chromatic aberration, no 

Chromosphere, 71-72, 93-94, 130- 
132, 138-139 

Circle, 171-173 

Clark, Alvan, & Sons, 117-118, 303 

Claudius, Emperor, 86 

Clavius (lunar crater), 199 

Clerk Maxwell, 237 

" Clouds" (of Aristophanes), lOl 

Clustering power, 325 

Clusters of stars, 300, 306, 314, 328 

Coal Sacks. See Holes in Milky 
Way 

Ccelostat, 119 

Coggia's Comet, 254 

Colour, production of, in telescopes, 
109-111, 115, 121 

Collision of comet with earth, 345- 
346 ; of dark star with sun, 346- 
348 ; of stars, 285, 312 

Columbus, 103 

Coma Berenices (constellation), 
307. 316 

Comet, first discovery of by photo- 
graphy, 258 ; first orbit calcu- 
lated, 255 ; first photograph of, 
257-258; furthest distance seen, 
258 ; passage of among satellites 
of Jupiter, 250 ; passage of earth 
and moon through tail of, 257, 346 



Comet of 1000 A.D., 262; 1066, 
262-264; 1680, 255, 265; 181 1, 
254-255; 1861, 254, 257, 346; 
1881, 257-258; 1882, 251, 258, 
291 ; 1889, 258; 1907, 258 

Comets, 27-28, 58, Chaps. XIX. 
and XX., 345-346 ; ancient view 
of, 259-261 ; captured, 251-253 ; 
Chinese records of, 83-84 ; com- 
position of, 252 ; contrasted with 
planets, 247 ; families of, 251-252, 
256 ; meteor swarms and, 274 ; 
revealed by solar eclipses, 95-96 ; 
tails of, 141, 182, 248, 252-254 

Common, telescopes of Dr. A. A., 
118 

Conjunction, 209 

Constellations, 105, 278-279, 285, 
289 

Contraction theory of sun's heat, 
128-129, 335 

Cook, Captain, 154 

Cooke, 118 

Copernican system, 20, 107, 149, 
170-173, 279, 280 

Copernicus, 20, 108, 149, 158, 
170-172, 236 

Copernicus (lunar crater), 200, 204 

Copper, 145 

Corder, H., 144 

Corona, 70-72, 90, 92-97, 132, 140- 
141, 270; earliest drawing of, 91 ; 
earliest employment of term, 90 ; 
earliest mention of, 86 ; earliest 
photograph of, 93 ; illumination 
given by, 71 ; possible change in 
shape of during eclipse, 96-98 ; 
structure of, 142-143 ; variations 
in shape of, 141 

Corona Borealis (constellation), 295 

Coronal matter, 142 ; streamers, 
95-96, 141-143 

Coronium, 133, 142, 317 

Cotes, 91 

Coude, equatorial, 119 

Cowell, P. H., 255, 264 

Crabtree, 152 

Crape ring of Saturn, 236-237 

Craterlets on Mars, 220 

Craters (ring-mountains) on moon, 
197-205, 214, 340; suggested 
origin of, 203-204, 214 



353 



Index 



Crawford, Earl of, 94 

Crecy, supposed eclipse at battle of, 
88-89 

Crescent moon, 183, 185 

Crommelin, A. C. D,, 255, 264 

Crossley Reflector, 118, 315-316 

Crown glass, 115 

Crucifixion, darkness of, 86 

Crucis, a (Alpha), 298 

Crux, or "Southern Cross" (con- 
stellation), 298-299, 323 

Cycle, sunspot, 136-137, 141, 143- 
144 

Cygni, 61, 173, 280 

Cygnus, or the Swan (constella- 
tion), 295, 325 

Daniel's Comet of 1897, 258 

Danzig, 1 1 1 

Dark Ages, 102, 107, 260 

Dark eclipses of moon, 65, 102- 

103 
Dark matter in space, 323 
Dark meteors, 275-276 
Dark stars, 309-310, 312, 323, 

346-347 
" Darkness behind the stars," 325 
Darwin, Sir G. H., 339 
Davis, 94 
Dawes, 236 

Dearborn Observatory, 303 
Death from fright at eclipse, 7^ 
Debonnaire, Louis le, 88, 261 
Deimos, 223 
Deity, symbol of the, 87 
" Demon star." See Algol 
Denebola, 296 
Denning, W. F., 269 
Densities of sun and planets, 39 
Density, 38 
Deslandres, 140 

Diameters of sun and planets, 31 
Disappearance of moon in lunar 

eclipse, 65, 102-103 
Disc, 60 
" Disc " theory. See " Grindstone " 

theory 
Discoveries, independent, 236 
Discovery, anticipation in, 236-237; 

indirect methods of, 120 
"Dipper," the, 291; the "Little," 

294 



Distance of a celestial body, how 
ascertained, 56-58 ; of sun from 
earth, how determined, 151, 211 

Distances of planets from sun, 47 

Distances of sun and moon, rela- 
tive, 68 

Dog, the Greater. See Canis Major ; 
the Lesser, see Canis Minor 

"Dog Star," 289, 297 

Dollond, John, 115-116 

Donati's Comet, 254, 257 

Doppler's method, 125, 136, 282, 
301-302 

Dorpat, 117 

Double canals of Mars, 2 14-21 5, 
218-220 

Double planet, earth and moon a, 
189 

Double stars, 300 

Douglass, 233 

"Dreams, Lake of," 197 

Dumb-bell Nebula, 316 

Earth, 20, 22, 31, 39, 48, 64, 
Chap. XV., 267 ; cooling of, 343 ; 
diameter of, 31 ; interior of, 166; 
mean distance of from sun, 47 ; 
rigidity of, 181 ; rotation of, 30, 
33, 161-165, 170; shape of, 165; 
"tail" to, 182 

" Earthlight," or " Earthshine," 
186 

Earth's axis, Precessional move- 
ment of, 175-177, 295, 298- 
299 

Earth's shadow, circular shape of, 
64, 160 

Eclipse, 61 

Eclipse knowledge, delay of, 74 

Eclipse party, work of, 73 

Eclipse of sun, advance of shadow 
in total, 69 ; animal and plant 
life during, 71 ; earliest record of 
total, 84 ; description of total, 
69-73 ; duration of total, 69, 72; 
importance of total, 68 

Eclipses, ascertainment of dates of 
past, 74 ; experience a necessity 
in solar, 73-74 : of moon, 63-65, 
Chap. IX., 203; photography in, 
93 ; prediction of future, 74 ; re- 
currence of, 74-80 



354 



Index 



Eclipses of sun, 25, 65-74, Chap, 
VIII., 201-202, 234 ; 1612 A.D., 
90; 1715, 88, 91; 1724,88,91; 
1836, 92 ; 1842, 92-93; 1851, 81, 
93; 1868, 93; 1870, 94; 1871, 
94; 1878, 95; 1882, 95; 1883, 
95-96; 1893, 95-96; 1896, 96, 
99; 1898, 96, 98; 1900, 97; 
1905. 75-76, 80-81, 97-98 ; 1907, 
98; 1908, 98; 1914, 99; 1927, 
92, 99-100 

Eclipses, Past and Future, 340 

Egenitis, 272 

Electric furnace, 128 

Electric light, spectrum of, 1 22 

Elements composing sun, 144-145 

Ellipses, 32, 6&, 172-173, 177- 
178 

Elliptic orbit, ^^, 177 

Ellipticity, 32 

Elongation, Eastern, 147, 149 ; 
Western, 147, 149 

Encke's Comet, 253, 256 

" End of the World," 342 

England, solar eclipses visible in. 
87-88, 91-92 

Epsilon, (e) Lyrse, 302 

Equator, 48 

Equatorial telescope, 226 

Equinoxes. See Precession of 

Eros, 210-211, 223, 226-227; dis- 
covery of, 24, 210, 227 ; impor- 
tance of, 211; orbit of, 32, 37, 
210, 336 

Eruptive prominences, 139 

Esclistre, 89 

Ether, 322-323, 331-332 

Europa, 233, 235 

Evans, J. E., 219 

Evening star, 149-150, 241 

Everest, Mount, 200 

Evershed, 182 

Eye-piece, no 

Fabricius, 307 
Faculse, 136, 143 
Fauth, 205 
Faye, 335 

Fi7i dti Monde, 346 
First quarter, 183 
" Fixed stars," 280 
Flagstaff, 215-216, 220 



Flammarion, Camille, 346 

Flamsteed, 90 

" Flash spectrum," 137 

"Flat," 112 

Flint glass, 115 

Focus, 66, 177 

" Forty-foot Telescope," 115 

Foster, 102 

Fraunhofer, 117 

French Academy of Sciences, 115 

Froissart, 89 

" Full moon " of Laplace, 190 

Galaxy. See Milky Way. 

Galilean telescope, 109 

Gahleo, 55, 109, 172, 197, 206, 

232-235, 242 
Galle, 24, 211, 244 
Ganymede, 233-234 
Gas light, spectrum of, 122 
Gegenschein, 181-182 
" Gem " of meteor ring, 271 
Gemini, or the Twins (constellation), 

22, 296-297 
Geminorum, ^(Zeta), 304 
Geometrical groupings of stars 

292 
"Giant" planet, 230, 238-239 
Gibbous, 183, 185 
Gill, Sir David, 211, 258, 291, 317- 

318 
Gold, 145 
Goodricke, 307 
Gore, J. E., 63, 285, 303, 307 308, 

310, 323-324. 331- 337, 34 
Granulated structure of photo- 
sphere, 134 
Gravitation (or gravity), 39, 41-45, 

128, 306 
Greek ideas, 18, 158, 161-162, 171, 

186, 197 
Green (rays of light), 121 
Greenwich Observatory, 143-144, 

232, 255, 303 
Gregorian telescope, 113- 114 
Grimaldi (lunar crater), 199 
" Grindstone " theory, 319-322 
" Groombridge, 1830," 281-282, 

326, 330 
Groups of stars, 306-307 
Grubb, Sir Howard, 118 
Gulliver's Travels, 224 



355 



Index 



Hale, G. E., 119, 140 

Half moon, 183, 185 

Hall, Asaph, 223 

Hall, Chester Moor, 115 

Halley, Edmund, 91, 255, 264-265, 

306 
Halley's Comet, 255, 264-265 
Haraden Hill, 91 
Harvard, 118, 302 
Harvest moon, 190-192 
Hawaii, 221 
Heat rays, 127 
Heidelberg, 226, 232 
Height of lunar mountains, how 

determined, 201 
Height of objects in sky, estimation 

of, 196 
Helium, 138, 145, 182 
Helmholtz, 128, 335 
Hercules (constellation), 295 
Herod the Great, 101-102 
Herodotus, 84 
Herschel, A. S., 269 
Herschel, Sir John, 92, 322 
Herschel, Sir William, 22, 36, 1 14- 

115, 204, 213, 235, 283, 292, 

308, 319-320, 326-328 
Herschelian telescope, 114, 119 
Hesper, 109 
Hesperus, 150 
Hevelius, iii 
Hezekiah, 85 
Hi, 83 
Hindoos, 18 

Hipparchus, 106, 177, 290, 311 
Ho, 83 

Holes in Milky Way, 321-323 
Holmes, Oliver Wendell, 213 
Homer, 223 
Horace, Odes' of, 106 
Horizon, 159 
Horizontal eclipse, 169 
Horrox, 44, 151-152 
Hour Glass Sea, 212 
Huggins, Sir William, 94, 125, 317 
Humboldt, 270 
*' Hunter's moon," 192 
Huyghens, 111-I12, 240, 242-243 
Hyades, 296-297, 307 
Hydrocarbon gas, 254 
Hydrogen, 94, 131, 138, 140, 144, 

156, 182, 254 



Ibrahim ben Ahmed, 270 
Ice-layer theory: Mars, 219; moon, 

205, 219 
Illusion theory of Martian canals, 219 
Imbrium, Mare, 197 
Inclination of orbits, 36-37 
Indigo (rays of light), 121 
Inferior conjunction, 147, 149 
Inferior planets, 20, 22 ; Chap. XIV., 

229 
Instruments, pre - telescopic, 106- 

107, 172 
International photographic survey 

of sky, 290-291 
Intra-Mercurial planet, 25-26 
Introduction to Astrono??iy, 31 
Inverted view in astronomical tele- 
scope, 116-I17 
lo, 233-234 
Iridum, Sinus, 197 
Iron, 145, 254 
Is Mars Habitable? 221 

Jansen, 108 

Janssen, 94, 236, 258 

Japetus, 240 

Jessenius, 89 

Job, Book of, 299 

Johnson, S. J., 103, 340 

Josephus, loi, 262 

Juno, 225 

Jupiter, 20, 22-23, 3i> 34> 37, 42, 
227-228, 230-236, 241, 272, 311; 
comet family of, 251-253, 256; 
discovery of eighth satellite, 26, 
232 ; eclipse of, by satellite, 234 ; 
without satellites, 234-235 

Jupiter, satellites of, 26, 62, 108, 
189, 232-235 ; their eclipses, 
234-235 ; their occultations, 62, 
234 ; their transits, 62, 234 

Kant, 334 

Kapteyn, 284, 313 

Keeler, 315, 337 

Kelvin, Lord, 129 

Kepler, 44, 152, 172, 237,242, 245, 

253, 311 
Kinetic theory, 156, 202, 212, 226, 

231, 239, 336 
King, L. W., 84 
Knowledge, 87 



356 



Index 



Labrador, 97 

Lacus Somniorum, 197 

" Lake of Dreams," 197 

Lalande, 244, 283 

Lampland, 215, 219 

Langley, 95, 127 

Laplace, 190, 333 

Laputa, 224 

Le Maire, 115 

Le Verrier, 24, 236, 243-244, 275 

Lead, 145 

Leibnitz Mountains (lunar), 200 

Leo (constellation), 270, 295-296 

Leonids, 270-272, 274-275 

Lescarbault, 25 

Lewis, T., 303 

Lexell's Comet, 250 

Lick Observatory, 31, 98, 117- 1 18, 

215, 232, 303, 305, 315 ; Great 

Telescope of, 117, 215, 237 
" Life" of an eclipse of the moon, 

80; of the sun, 77-78 
Life on Mars, Lowell's views, 217- 

218; Pickering's, 221 ; Wallace's, 

221-223 
Light, no extinction of, 322-324 ; 

rays of, 127 ; velocity of, 52, 

235-236 ; white, 121 
"Light year," 53, 280 
Lindsay, Lord, 94 
Linne (lunar crater), 205 
Liouville, 190 
Lippershey, 108 
Liquid-filled lenses, 116 
Locksley Hall, 296; Sixty Years 

After, 109 
Lockyer, Sir Norman, 73, 94, 236, 335 
Loewy, 119, 206 
London, eclipses visible at, 87-88, 

91-92 
Longfellow, 2>^ 
Lowell Observatory, 215, 219, 233- 

234 
Lowell, Percival, 155, 212-213, 

215-221 
Lucifer, 150 
Lynn, W. T., 219, 263 
Lyra (constellation), 177, 283, 294- 

295, 307, 315. 347 



Madler, 206, 284 
Magellanic Clouds, 317 



Magnetism, disturbances of terres- 
trial, 143, 283 

Magnitudes of stars, 287-289 

Major planets, 229-230 

" Man in the Moon," 197 

Alamial of Astronomy, 166 

Maps of the moon, 206 

Mare Imbrium, 197 

Mare Serenitatis, 205 

Mars, 20, 22-23, 31-32, 34, 37, 109, 
155, 210-225, 234; compared 
with earth and moon, 221, 225; 
polar caps of, 212-214, 216; 
satellites of, 26, 223-224 ; tem- 
perature of, 213, 216, 221-222 

Mass, 1%; of a star, how determined, 

305 
Masses of celestial bodies, how 

ascertained, 42 ; of earth and 

moon compared, 42 ; of sun and 

planets compared, 39 
Maunder, E. W., 87, 143, 219 
Maunder, Mrs., E. W., 96, 144 
Maxwell, Clerk, 237 
Mayer, Tobias, 206, 283 
McClean, F. K., 98 
Mean distance, 46 
" Medicean Stars," 232 
Mediterranean, eclipse tracks across, 

94, 97 
Melbourne telescope, 1 18 
Melotte, P., 232 
Mercator's Projection, 80-81 
Mercury (the metal), 145 
Mercury (the planet), 20, 22, 25- 

26, 31-32, 34, 37, Chap. Xiy. ; 

markings on, 156; possible 

planets within orbit of, 25-26; 

transit of, 62, 151, 154 
Metals in sun, 145 
Meteor swarms, 268-269, 271, 274- 

275 
Meteors, 28, 56, 167, 259, Chap. 
XXI. 

Meteors beyond earth's atmosphere, 

275-276 
Meteorites, 276-277 
Meteoritic Hypothesis, 335 
Metius, Jacob, 108 



Middle Ages, 102, 260, 264 
Middleburgh, 108 



357 



Index 



Milky Way (or Galaxy), 285, 299, 
311,317,319-327; penetration of, 
by photography, 325 
Million, 47, 51-52 
Minor planets. See Asteroids. 
Mira Ceti, 307-308 
" Mirk Monday," 89 
Mirror (speculum), iii, 116 
Mizar, 294, 302 
Monck, W. H. S., 275 
Mongol Emperors of India, 107 
Moon, 26, Chap. XVI. ; appearance 
of, in lunar eclipse, 6$, 102 - 
103 ; diameter of, 189 ; distance 
of, how ascertained, 58 ; dis- 
tance of, from earth, 48 ; full, 
63, 86, 149, 184, 189, 190, 206; 
mass of, 200, 202 ; mountains on,, 
197-205 ; how their height is de- 
termined, 201 ; movement of, 40- 
42; new, 86, 149, 1S3, 185; origin 
of, 339-341 ; plane of orbit of, 
63 ; possible changes on, 204- 
205, 221; "seas" of, 197, 206; 
smallest detail visible on, 207 ; 
volume of, 200 
Morning star, 149-150, 241 
Moulton, F. R., 31, ii8, 128, 302, 

335, 337 
Moye, 154 
Multiple stars, 300 
Musa-ben-Shakir, 44 
Mythology, 105 



Neap-tides, 179 

Nebulae, 314-318, 328, 335' 345 ; 

evolution of stars from, 317-318 
Nebular Hypothesis of Laplace, 

333-338 

Nebular hypotheses, Chap. XXVII. 

Nebulium, 317 

Neison, 206 

Neptune, 20, 25, 31, 34, 37, 243- 
246, 249, 252, 274, 304 ; dis- 
covery of, 23-24, 94, 210, 236, 
243-244; Lalande and, 244; 
possible planets beyond, 25, 252 ; 
satellite of, 26, 245 ; "year" in, 
35-36 

"New" (or temporary), stars, 310- 

314 



Newcomb, Simon, 181, 267, 281, 
^ 324, 326-327, 329 
Newton, Sir Isaac, 40, 44, 91, 

111-113, 115, 165, 172, 237, 

255 
Newtonian telescope, 112, 114, 116, 

119 
Nineveh Eclipse, 84-85 
Nitrogen, 145, 156,' 166, 346 
Northern Crown, 295 
Nova Aurigae, 311 
Nova Persei, 312-314 
Novae. See New (or temporary) 

stars 
Nubeculae, 317 



" Oases " of Mars, 216, 220 

Object-glass, 109 

Oblate spheroid, 165 

Occultation, 61-62, 202, 296 

Olaf, Saga of King, 88 

Olbers, 227, 253, 256, 271 

" Old moon in new moon's arms," 

185 
Olmsted, 271 
Omicron (or "Mira") Ceti, 307- 

308 
Opposition, 209 
" Optick tube," 108-109, 232 
Orange (rays of light), 121 
Orbit of moon, plane of, (i^ 
Orbits, 32, 36-37, 66, 150, 157 
Oriental astronomy, 107 
Orion (constellation), 195, 279, 

296-297, 316; Great Nebula in, 

316, 328 
Oxford, 139 
Oxygen, 145, 156, 166, 346 



Pacific Ocean, origin of moon 

in, 339 
Palitzch, 255 
Pallas, 225, 227 
Parallax, 57, 173, 280, 305, 320, 

326 
Pare, Ambrose, 264-265 
Peal, S. E., 205 
Peary, 277 

Pegasus (constellation), 306 
I Penumbra of sunspot, 135 



358 



Ind 



ex 



Perennial full moon of Laplace, 190 

Pericles, 84 

Perrine, C. D., 232-233, 315 

Perseids, 270, 273-275 

Perseus (constellation), 273, 279, 

307. 312 
Phases of an inferior planet, 149, 
160 ; of the moon, 149, 160, 
183-185 
Phlegon, Eclipse of, 85-86 
Phobos, 223 
Phoebe, retrograde motion of, 240, 

250, 336 
Phosphorescent glow in sky, 323 
Phosphorus (Venus), 150 
Photographic survey of sky, inter- 
national, 290-291 
Photosphere, 130-131, 134 
Piazzi, 23 

Pickering, E. C, 302 
Pickering, W. H., 199, 205-206, 

220-221, 240, 339-341 
Pictor, "runaway star" in con- 
stellation of, 281-282, 320, 330 
Plane of^rbit, 36, 150 
Planetary nebulae, 245, 315 
Planetary and Stellar Studies^ 331 
Planetesimal hypothesis, 337-338 
Planetoids. See Asteroids 
Planets, classification of, 229 ; con- 
trasted with comets, 247 ; in 
Ptolemaic scheme, 171 ; relative 
distances of, from sun, 31-32 
Plato (lunar crater), 198 
Pleiades, 296-297, 284, 307 
Pliny, 169, 260 
Plough, 284, 291-296, 302 
Plutarch, 86,89, 181, 169 
"Pointers," 292 
Polaris. See Pole Star 
Pole of earth, Precessional move- 
ment of, 176-177, 295, 298-299 
Pole Star, 33, 163, 177, 292-296, 

300-301 
Poles, 30, 163-164 ; of earth, speed 

of point at, 164 
Pollux, 282, 297 
Posidonius, 186 
Powell, Sir George Baden, 96 
Prsesepe (the Beehive), 307 
Precession of the Equinoxes, 177, 
295, 298-299 



Pre-telescopic notions, 55 

Primaries, 26 

Princess, The (Tennyson), 334 

Princeton Observatory, 258) 

Prism, 121 

Prismatic colours, ill, 121 

Procyon, 284, 290, 297, 303 

Prominences, Solar, 72, 93, 131, 

139-140, 143 ; first observation 

of, with spectroscope, 94, 140, 

236 
Proper motions of stars, 126, 281- 

285, 326, 329-330 
Ptolemseus (lunar crater), 198-199, 

204 
Ptolemaic idea, 319 ; system, 18, 

19, 158, 171-172 
Ptolemy, 18, loi, 171, 290, 296 
Puiseux, P., 206 
Pulkowa telescope, 117 
Puppis, v., 310 



Quiescent prominences, 139 



Radcliffe Observer, 139 

" Radiant," or radiant point, 269 

Radiation from sun, 130, 134 

Radium, 129, 138 

Rainbow, 121 

"Rainbows, Bay of," 197 

Rambaut, A., 139 

Ramsay, Sir William, 138 

Rays (on moon), 204 

Recurrence of eclipses, 74-80 

Red (rays of light), 121, 125, 127, 

130 
Red Spot, the Great, 230 
Reflecting telescope, III-116; 

future of, 119 
Reflector. See Reflecting telescope 
Refracting and reflecting telescopes 

contrasted, 118 
Refracting telescope, 109- 1 11, 

115-117; limits to size of, 119- 

120 
Refraction, 121, 168-169 
Refractor, See Refracting telescope 
Regulus, 290, 296 
Retrograde motion of Phoebe, 240, 

250, 336 



359 



Ind 



ex 



" Reversing Layer," 94, 130, 132, 

137-138 
Revival of learning, 107 
Revolution, 30 ; of earth around 

sun, 170-173 ; periods of sun 

and planets, 35 
Riccioli, 198 
Rice-grain structure of photosphere, 

134 
Rigel, 285, 297 
Rills (on moon), 204 
Ring-mountains of moon. See 

Craters 
•' Ring" nebulae, 315, 337 
" Ring with wings," 87 
Rings of Saturn, 108, 236-239, 241- 

243, 334 
Ritchey, G. W., 118 
Roberts, A. W., 308, 310 
Roberts, Isaac, 325 
"Roche's limit," 238 
Roemer, 235 

'Roman history, eclipses in, 85-86 
Romulus, 85 
Rontgen, 120 
Rosse, great telescope of Lord, 

117, 314 
Rotation, 30 ; of earth, 33, 161-165, 
170; of sun, 34, 125, 135-136, 
231 ; periods of sun and planets, 

35 
Royal Society of London, 90-91, 

III 
Rubicon, Passage of the, 85 
" Runaway " stars, 281, 326, 330 



Sagittarius (constellation), 316 

Salt, spectrum of table, 122 

Samarcand, 107 

*'Saros," Chaldean, 76-78, 84 

Satellites, 26-27, 37 

Saturn, 20, 22, 34, 37, 108, 236- 
243, 258 ; comet family of, 252 ; 
a puzzle to the early telescope 
observers, 241-243 ; retrograde 
motion of satellite Phoebe, 240, 
250, 336 ; ring system of, 241 ; 
satellites of, 36, 239-240; shadows 
of planet on rings and of rings 
on planet, 237 

Schaeberle, 95-96, 303, 316 



Schiaparelli, 155, 214, 223 

Schickhard (lunar crater), 199 

Schmidt, 206 ^ 

Schonfeld, 290 

Schuster, 95 

Schwabe, 136 

Scotland, solar eclipses visible in, 
89-90, 92 

Sea of Serenity, 205 

" Sea of Showers," 197 

" Seas " of moon, 197, 206 

Seasons on earth, 174-175 ; on Mars, 
211 

Secondary bodies, 26 

Seneca, 95, 260 

Septentriones, 291 

Serenitatis, Mare, 205 

" Seven Stars," 291 

" Shadow Bands," 69 

Shadow of earth, circular shape of, 
62-64 

Shadows on moon, inky blackness 
of, 202 

Shakespeare, 259, 293 

Sheepshanks Telescope, 119 

" Shining fluid " of Sir W. Herschel, 
328 

" Shooting Stars." ^'i?^ Meteors 

Short (of Edinburgh), 114 

" Showers, Sea of," 197 

Sickle of Leo, 270-271, 296 

Siderostat, 118 

Silver, 145 

Silvered mirrors for reflecting tele- 
scopes, 116 

Sinus Iridum, 197 

Sirius, 280, 282, 284-285, 288- 
290, 297, 303-304, 320; com- 
panion of, 303 ; stellar magnitude 
of, 289 

Size of celestial bodies, how ascer- 
tained, 59 

Skeleton telescopes, no 

Sky, international photographic 
survey of, 290-291 ; light of the, 
323 

Slipher, E. C, 213, 222 

Smithsonian Institution of Washing- 
ton, 98 

Snow on Mars, 213 

Sodium, 122, 124, 254 

Sohag, 95 



360 



Ind 



ex 



Solar system, 20-21, 29-31 ; centre | 
of gravity of, 42 ; decay and death 
of, 344 

Somniorum, Lacus, 197 

Sound, 125, 166, 331 

South pole of heavens, 163, 285, 
298-299 

Southern constellations, 298-299 
uthern Cross. See Crux 

^pace, 328 

Spain, early astronomy in, 107 ; 
eclipse tracks across 93, 97- 
98 

Spectroheliograph, 140 

Spectroscope, 120, 122, 124-125, 
144-145, 212, 231 ; prominences 
first observed with, 94, 140, 236 

Spectrum of chromosphere, 132- 
133 ; of corona, 133 ; of photo- 
sphere, 132 ; of reversing layer, 
132, 137; solar, 122-123, 127, 
132 

Speculum, ill, 116; metal, 112 
V Spherical bodies, 29 

Spherical shape of earth, proofs of, 
158-161 

Spherical shapes of sun, planets, 
and satellites, 160 

Spiral nebulae, 314-316, 337-338 

Spring balance, i66 

Spring tides, 192 

Spy-glass, 108 

" Square of the distance," 43-44 

Stannyan, Captain, 90 

Star, mass of, how determined, 305 ; 
parallax of, first ascertained, 173, 
280 

Stars, the, 20, 124, 126, 278 et seq. ; 
brightness of, 287, 320 ; distances 
between, 326-327 ; distances of 
some, 173, 280, 320 ; diminution 
of, below twelfth magnitude, 324 ; 
evolution of, from nebulae, 317- 
318 ; faintest magnitude of, 288 ; 
number of those visible altogether, 
324 ; number of those visible to 
naked eye, 288 

" Steam cracks," 221 

Steinheil, 118 

Stellar system, estimated extent of, 
325-327 ; an organised whole, 
327 ; limited extent of, 322-328, 



330 ; possible disintegration of, 

329 

Stiklastad, eclipse of, 88 

Stone Age, 285 

Stoney, G. J., 202, 222 

Stonyhurst Observatory, 100 

Story of the Heavens^ 27 1 

Streams of stars, Kapteyn's two, 284 

Stroobant, 196 

Stukeley, 91 

Sulphur, 145 

Summer, 175, 178 

Sun, Chaps XII. and XIII. ; as a 
star, 124, 278, 289 ; as seen from 
Neptune, 246, 304; chemical com- 
position of, 144-145 ; distance 
of, how ascertained, 151, 211 ; 
equator of, 135-136, 139; gravi- 
tation at surface of, 129, 138-139 ; 
growing cold of, 343-344 ; mean 
distance of, from earth, 47, 211; 
motion of, through space, 282- 
286, 326 ; not a solid body, 136 ; 
poles of, 136; radiations from, 
130; revolution of earth around, 
170-173 ; stellar magnitude of, 
288-289 ; variation in distance of, 
66, 178 

Sunspots, 34, 125, I34-I37» 140- 
141, 143-144, 308; influence of 
earth on, 144 

Suns and possible systems, 50, 286 

Superior conjunction, 147-149 

Superior planets, 22, 146, 209-210, 
229 

Swan (constellation). See Cygnus 

Swift, Dean, 224 

"Sword" of Orion, 297, 316 

Syrtis Major. See Hour Glass Sea 

^'■Systematic Parallax," 326 

Systems, other possible, 50, 286 



Tails of comets, 182 

Tamerlane, 107 

Taurus (constellation), 103, 296- 

297, 307 
" Tears of St. Lawrence," 273 
Tebbutt's Comet, 257-258 
Telescope, 33, 55, 107-108, 149 ; 

first eclipse of moon seen through, 

104 ; of sun, 90 



361 



Index 



71-72 

; track of, 66 
minor planet, 



226- 



Telescopes, direct view reflecting, 
114; gigantic, iii; great con- 
structors of, 1 17 -1 18; great 
modern, 11 7- 118 

Tempel's Comet, 274 

Temperature on moon, 203 ; of sun, 
128 

Temporary (or new) stars, 310- 
3H 

Tennyson, Lord, 109, 296, 334 

Terrestrial planets, 229-230 

Terrestrial telescope, 117 

Thales, Eclipse of, 84 

Themis, 240 

"Tidal drag," 180, 188, 208, 344 

Tide areas, 179-180 

Tides, 178-180, 338-339 

Time Machine, 344 

Tin, 145 

Titan, 240 

Titius, 245 

Total phase, 

Totality, 72 

Trail of a 
227 

Transit, 62, 150-154; of Mercury, 
62, 151, 154; of Venus, 62, 151- 
152, 154, 211 

Trifid Nebula, 316 

Triple stars, 300 

Tubeless telescopes, iio-iii, 243 

Tubes used by ancients, no 

Tuttle's Comet, 274 

Twilight, 167, 202 

Twinkling of stars, 168 

Twins (constellation). See Gemini 

Tycho Brahe, 290, 311 

Tycho (lunar crater), 204 



Ulugh Beigh, 107 
Umbra of sunspot, 134-135 
Universe, early ideas concerning, 

17-18, 158, 177, 342 
Universes, possibility of other, 330- 

331 
Uranus, 22-24, 3i» 210, 243, 245, 
275; comet family of, 252; dis- 
covery of, 22, 210, 243; rotation 
period of, 34, 245 ; satellites of, 
26, 245 ; "year" in, 35-36 



Ursa Major (constellation), 279, 
281, 291, 295, 314; minor, 177, 
279, 293-294 

Ursse Majoris, (^) Zeta. See Mizar 



Variable stars, 307-310 

Variations in apparent sizes of sun 
and moon, 67, 80, 178 

Vault, shape of the celestial, 194- 
196 

Vega, 177, 278, 280, 282-283, 285, 
290, 294, 302, 307, 323 

Vegetation on Mars, 221, 217-218; 
on moon, 205 

Venus, 20, 22, 31, 71, 90, 108-109, 
III, Chap, XIV., 246, 311 ; rota- 
tion period of, 34, 155 

Very, F. W., 314 

Vesta, 225, 227 

Violet (rays of light), 121-122, 125 

Virgil, 19 

Volcanic theory of lunar craters, 
203-204, 214 

Volume, 38 

Volumes of sun and planets com- 
pared, 38-39 

"Vulcan," 25 



Wallace, A. R., on Mars, 220-223 

Water, lack of, on moon, 201-202 

Water vapour, 202, 213, 222 

Wargentin, 103 

Warner and Swasey Co., 117 

Weather, moon and, 206-207 

Weathering, 202 

Webb, Rev. T. H., 204 

Weight, 43, 165-166 

Wells, H. G., 344 

Whale (constellation). See Cetus 

Whewell, 190 

Willamette meteorite, 277 

Wilson, Mount, 118 

Wilson, W. E., 313 

" Winged circle " (or " disc "), 87 

Winter, 175, 178 

Witt, 227 

Wolf, Max, 226-227, 232 

Wright, Thomas, 319, 334 

Wybord, 89 



362 



Index 



Xenophon, ioi 



Year. 35 

" Year " in Uranus and Neptune, 

35-3^ 

Year, number of eclipses in a, 68 

" Year of the Stars," 270 

Yellow (rays of light), 121-122, 124 



Yerkes Telescope, Great, 117,^303 
Young, 94, 137, 166 



Zenith, 174 

Zinc, 145 

Zodiacal light, i8l 

Zone of asteroids, 30-31, 227' 



THE END 



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